What calculator are you using? Can you attach it to this thread? Is there one on this forum as a permanent link? I can say that I've used the one I attached for 2 different builds to figure out speedo drives and it has worked well. Also, both of the vehicles had multiple transmissions and rears and the calculator was accurate, for what it's worth.
If there isn't one in a permanent link/sticky here, then there probably ought to be. IMO