Here's how I set my posts. Since I work by myself or with a day laborer I usually use a string if the lines are short. If they're longer what I will do is temporarily stand up plumb marker posts. One at each end and one in the middle. The middle one I place by stabbing it into the ground. I go back to the closest end and then sight down the side. I keep moving the middle post in and out until when I sight down the side of the end post towards the other end post I can't see it because of the middle post.
I will then start at one corner and with upside down paint if the ground is hard or with a claw hammer if it's soft I'll mark off lines perpendicular to the fence line on eight foot centers.
If when I get to the other end I'll stop about thirty feet away and measure. I will then try to divide that measurement so that I have less than eight but an even number. Sometimes I have to come back more than thirty feet to get that number but what I'm after is not a short section at the corner. That screams to even the varmints that either a rookie or someone who didn't have much pride set those posts!
Once I have the eight foot centers marked I pick up a pair of post hole diggers. I hold them from the tops of the handles over the mark. I look down one side. I move the diggers making sure I'm holding them by the handles high so that they're hanging plumb in and out until the edge of the diggers lines up with the center marker post and the one at the far end. When they line up just perfect. You'll know. I drop the diggers and they will leave a mark. If I am using paint I will mark across my eight foot center mark with the paint. I do this over each mark. Now when I hit the marker post I turn around and sight back to the other end post. What is important is if I sighted down the north side of the line even though I'm looking back the other way I make sure I'm sighting down the north side.
I dig my holes. Now what you might want to keep in mind is the width of your posts and allow for the placement of the center of your auger accordingly. In other words if you're posts are two and a half inches o.d. and you're going to leave your original marker posts up to help with your setting (good idea) you might want to center your auger an inch over so that it's center of the post line and not center of the north edge of the fence line.
I set my corner posts. I set them one inch higher than nominal fence height. I use a marker that will contrast with the post to mark down the diameter of the toprail plus the one inch. I make a nice horizontal line that is easy to see at a distance. I knock down the center marker post and get it out of my way.
I go to the second post in line from the end I'm going to start setting. I line it up the same way I did the marking post.
BTW the posts are always lined up with a torpedo level holding them plumb!
And don't use the magnetic side!
The devil will get you for that!
Actually what will happen is if you're not triple careful you'll pull your post out of plumb removing the level.
I will usually put a little mixed concrete in that second post to hold it in place while I set it. Again, plumb it up, check it, move it plumb, check it, etc.
When it's dead on where when you look down the side you can't see the other end post unless you move your eye a half inch off line you're doing good.
Fill the hole with concrete. Since you're doing livestock fence I suggest leaving the concrete down four to six inches from the top of the hole. The livestock will walk the fence line. If the path they wear down erodes the dirt around your post and the concrete is higher then you have a potential hazard to them. And if they don't wear it down and you weed eat it will save you string hitting the smooth pipe versus the ragged concrete.
Now you have the second post set for line. Go back to your near end post. Hold your plumb for level at your line you made. Sight over the top of the level towards your other end. What you're wanting to see is the bottom of your saddle and just barely your corresponding mark on the other end post.
Did I mention you presaddle all your posts? I should have.
You move up and down the second post until it's perfect. Each time you move it you check if for line.
I know it sounds busy but you only have to do this one time a line.
Once it's perfect then you go to post between your second post and your near corner or end. Holding that post plumb with your torpedo level you move it in and out of line until you can see either nothing of the far end post or just a hair of it. You put your concrete in the hole. Now you double check your post for line and pull it up for height. What you're looking for is to barely see that horizontal line over the bottom of the saddle of the post your'e setting. When it's perfect you step around the post and look back at the other end to verify it's perfect that direction too.
When that one's perfect then you're home free. All the rest of it's pure work plain and simple. You pick up your third line post and sight over the saddle when it's plumb and in line towards your near corner or end. When it's perfect you step back to your previous post and double check it by looking down over it to the far end.
This might sound like too much work and too complicated. I have folks all the time tell me that it's too hard when it's so easy to stretch a string and set all the posts high. Then you come back and stretch another string tight mark your posts and cut them all with a torch and then saddle them with the torch and a sure cut.
You can usually see when they've done that. The fence will have a bow because any wind that blows will catch string. It also so sags so you will see where the middle of the section is slightly lower than the ends.
I spend fifty percent longer setting my posts this way than they do with the string. But when it's time to come back welding in I have perfect saddles dead in line for height and line. I just lay in my top rail and weld it in. I'm done before they are and my job looks a hundred times better. If the customer wouldn't know the difference then they're not the kind of customer I want anyway.
I like working with picky people. Picky people appreciate quality. Sometimes you have to educate them but generally they're capable of learning too.
I once was just cleaning up from setting a line about four hundred feet long. An old boy drove up, got out of his truck, looked down the saddles, and shook his head. I don't think I ever really convinced him that I'd done it all by eye. To this day I'm sure he'll tell you I am the best man he ever seen with a string.
But setting posts this way is a good metaphor on life. The whole secret to a perfectly straight line is making sure each post is perfect and then leaving it to go to the next. It's sorta funny how if you've made sure each one is perfect then when you get to the end it's all perfect and it was done one at a time.
Sorry for the lengthy diatribe. But there's few things finer than looking down a straight line of posts and knowing that you're the luckiest guy on the planet.