Consider how pre-setting accounts for future sagging under its own weight. It locks in that correct geometry before gravity does its thing. You basically build the compensation right into the initial setup.
Getting a gate wrong on the first try is pretty normal for DIY stuff. I did the same thing and had to rehang the whole thing after it drooped. What worked for me was using a temporary brace to hold the gate in the raised position while I attached the hinges. That way, when I removed the brace, the gate settled into the perfect spot with just a bit of pre sag. It sounds like extra work, but it saves so much hassle later. Now I check the alignment with a level and a string line to make sure it's all square before locking everything down.