Yeah, that's basically it. Weigh your egg before you set it, calculate what weight it's supposed to end up at, then do your best to get it there!
I don't do average weights, and I've never plotted a graph. I just do a quick calculation and keep notes on a sheet of paper. Like, if the egg's starting weight is 75g and it's supposed to lose 13%, that would leave 87%. So the calculation is 75 x 0.87 = 65.25. You don't need to be mega accurate and I only use scales that weigh to the nearest gram anyway, so I round it up or down to which ever whole number is closer. So I'd be wanting this egg to end up at 65g, though if it ended up at 64g or 66g I wouldn't worry too much.
So this egg is supposed to lose about 10g. Once you know that, it's easy to keep track of how it's getting on. The way I do it is to candle and weigh on days 6 and 12 and then on day 18 just before lockdown. That divides the incubation period into three equal parts. Then I mentally divide the 10g by 3 as well, to get roughly 3 and a bit. So, the first time I weigh the egg on day 6, I'd be expecting it to be at about 72g, and the second time I weigh it on day 12 I'd be expecting it to be at about 68g. If it's not where I want it to be, I adjust the humidity either up or down.
I number each egg that I'm incubating and I also write the starting weight on the shell just in case I lose my notes. On my notes, I write down the starting weight of each egg, and also the weight I want it to end up at, just so it's there in front of me. It probably sounds more complicated than it really is. Here's an old page of notes from one of my hatches, in case I'm not explaining very well. This was a hatch where I was experimenting with hatching old eggs, as well as with disinfecting and washing them, which is why there's extra scribbles about that stuff. Normally I just write the number of each egg down and keep track of the weights...