don't forget to check your diagonals according to your drawing for square. Have somebody help you hold the tape to get an accurate measurement. Also, check to make sure your panels are level in the vertical direction as well. If you are further out at the top then the bottom it will throw off your measurements.
If your diagonals are good then set up a string line on your batter boards. The string line should tell you which panel is bowing on you.
I also highly highly recommend using a laser level and check every single panel for level in conjunction with your your bubble level. when I installed my pool I set a half inch by 1.5 foot pin at the finished height of my walls. I used that to setup my laser level every time I started working. It was a big enough pin I knew it wasn't going anywhere.
If you can't get it exact being off 3/4" won't affect your line placement. The liner will stretch into that no problem.
Diagonals within 1/4" both top and bottom as compared to manufacturer drawing. I'm an engineer and surveyor, so i set a 3/4" 2' long rod in ground for benchmark. Panels all dead level. string lines from the beginning, panels all right in line except for one end which is 3/4" further back. used a laser level the whole process. I sent an email to hydrapools so hopefully they will have some explanation. taking both sides of this pool back down is not something i'm looking forward to especially when i went to great lengths to get this right. I just don't see how i can squeeze 3/4" back into this wall. measured with 2 different tapes for verification. all wall panels plumb.