No, you've got the right idea. I've always wanted to have the guts to do one rolling a little or bows down, but most people would probably think it was a lousy model. i'd eyeball what looks right,never can tell if she's just been re coaled.
I put a big piece of tape around the hull with the bottom exposed and paint it black. Rough saw off the bottom and then as you say sand it flat. Unless you are showing her anchored in still water, the technique is forgiving because you can hide the cut with waves.
I was told, and agree, the best way to model the wake in whatever medium you choose is to study a bunch of photographs from above of any ships you can find that are as similar as possible. Wkaes are complex things, unexpected details. The results are much better than guessing.
This'll be a nice model.