Hello,
I would suggest to find literature on your mother language but here I present a formula from my language(Croatian) which I used in my final assignment. If you are interested in this Croatian literature it is called "Oberšmit - Ozubljenja i zupčanici" but I would greatly suggest you to read "Dudley's Handbook of Practical Gear Design and Manufacture" this book has every little detail of designing the gears. PS.Use google translator if you need translations