I think I found what you are referring too, it is by Don Wright, in The Book Of Buckskinning II, page 121, The Base Plug.
I try and stay away from math as much as possible. After the horn comes off the sizer, I measure the inside with an Inside/Outside caliper and turn from there, try fitting as I go until I get the fit I am looking for. Once it dries and comes off the sizer the inside should be round, turning a base to fit should be no problem from there. The outside of the horn may take some work to fit the base.
Tim C.