I agree that if you know exactly the distance from centerline to centerline, and you can judge the same exact difference when looking at a target 200 yds. away, then by all means it works. But if you're approximating any part of this, then it's going to change at varying distances. I suppose with enough calculations of outside diameter of the scope, barrel, it could be calculated pretty close, and maybe exactly for someone with the right equipment. Don't forget to include external taper of the barrel when doing all those calculations. And be sure whoever tapered it checked run out from the bore too!
I'd have a tough time estimating 3/4" at 200 yds., unless the scope used had pretty good magnification. Since my old guns get old scopes, then 6x is about as high power as my scopes get. Again, if it's for hunting purposes, then it's not a concern.
