Are there any sources (books of Else) to determine of as guide to figure out?
Because I didn't have any sort of muzzle-loading gun under my roof, or any close-by builders at the time, I laminated a pair of 2x4's and roughed out my design for drop, pull, and cast-off. I used a nail for trigger position, and added a bit of iron out front for weight and balance. I very roughly shaped the thing (wrist, cheek, butt) and used the seam between the 2x4's as my sighting reference. I'm pleased with what it showed me as I built those numbers into my first build.
Coming from the bicycle arena, I knew that "what fits most" didn't fit me. I built my bikes (could have hired it done) to get something that actually fit me properly. There is no factory bicycle frame with the geometry I need, only custom. Fitment in cycling is critical for best muscle recruitment/efficiency and handling.
Guns I don't find quite so critical, but that I was keenly aware of how much fantastic my fitted bikes rode compared to anything I'd ever had before (in standard frame geometries). So I wasn't about to miss an opportunity to be sure I had good fitment with my longrifle. Also it's much different when not using a rear sight and I'll be doubly careful laying out any smoothies I build. Measurements and formulas can give us jumping off points, but I'm still going to use a try-stock to double check it all.
Daryl is right, Drop is a styling component. You can't monkey with the drop a lot without seriously compromising the school/style of gun. There's one small company that makes BP guns that look "flat" to me because they use more modern geometries on their stocks--universally I suppose. Those guns don't interest me at all, they just don't look right to my eye and I'd rather deal with compromised fit than to carry a ML with modern stock shapes.