What is the calculation to determine speedo gear size for a A833?

Hello!
I've searched the forum but can't seem to find how to determine what size speedo gear to use after a tire and gear change. I"m running a stock A833 4 speed trans, rear tire height it 26.5, and the new gear ratio will be 4.10. How do you determine the correct speedo gear?
Thanks!
MJK