Anthony, to be honest, I do not worry about that. Horn is more consistent them most plastic dice sold commercially. It's amazing what kind of cavities you can find when cutting dice in two.
I had some dice made from bison horn using CNC grinders and laser cutters and they are no more accurate then the dice I made.
One can get special gamblers dice which are balanced to perfection. Never rolled more boring dice then those.
In the game I use these for, the dice result isn't all that important either, so I just try to get the densest and best looking pieces of the horn to make dice from. That means about 90% of the horn can only be used to make dice cups or jewelry of