I made a set of cables using aircraft cable with shrink tubing. I would put two nails into the court under the net on the outside edges so that I could simply do a diagonal/straight to get my outside corners. It made the striping job a one man operation that I could do in a couple of hours. I also used a 2 inch paint roller that I put screws in to prevent the roller from turning. I put the roller on a stick and used a squirt bottle to apply my line paint. This made the striping job easy.
Had to do 6 red clay every spring. I did the same but used the posts and scribed an arc at the laid out string line at the base across three courts. That got me square. Then measured the distance from corner to corner and split the difference
I would never drill a hole - that creates a weak spot in the surface. Put a piece of tape there and mark it. Once your strings are in place, remove the tape.
why not use existing net sockets, make a jig with a protruding pin, measure between, use your laptop/printout to allow for any variations, use relevant readings and with two tapes job done ! less time, less manpower, I thought you lot were top of your game