My reason for this post is I'm making my own buttplate. When I've bought pre-made buttplates the angle is already established. So, if I buy a Fowler plate, I then measure my drop at heel and my drop at the comb nose, and my buttplate angle just follows\fits that. I don't really have options to change it.
So, for this current build I think I'll bend the plate so I end up with the buttplate being 90 degrees from the barrel.