It really is a chore but, the only real way to meaure is to pour and mark, pour and mark.
Think of this way, since your doing by the gallon you'll only have to mark 15 times (if that). When I set up mine, I did it by quarts. 2 months later I installed a sight guage (HLT) and had to do it again.
I notch marked my brew paddle. Easy peezy. And when I am running off I place a piece of tape on the inside wall of the kettle as a marker so I don't have to keep standing up the paddle.