Questions about a 2 speed

[lincpimp]

Just received my Yusa on road car and was wondering what to do about the 2 speed center trans. I would like to keep the two speed and was wondering how to gear it. I know that mike makes a 2 speed adapter to use the losi mod1 screw-on gears. How do you determine which pinion gears to use together. I will list the stock gear sizes when I have a chance, but I understand that the two pinions have to be specific sizes for each to mesh correctly with the spur gears at the same time.