That’s where templates come in. Production shops use them – how else do you think they can be production shops?
For any project that I make that requires a series of arcs or a long diagonal line, I’ll create a template. Before I do that, I make sure I hone the part as perfect as I can get it, then trace around it on a piece of scrap.
Below are two templates cut form 3/4″ stock. I’ve labeled them on one side using a permanent pen. On the other side I made a couple of notes that apply to them. Cutoffs (the small ends leftover from crosscut stock) are great for templates such as these parts.
Most of the time for large project parts, I use scrap 1/4″ plywood, or even poster board. With 1/4″ ply, be sure to store the templates flat to minimize warping as much as possible.
I try to apply a “catch and release” program on scrap. If I think it’ll be large enough to use as a template, or even a spacer, I’ll hold on to it for a while. If not, I toss it out, or (depending on the species) try to save it for the firewood pile.