Take the axle sprocket divided by the countershaft sprocket and that's your gear ratio. Very helpful tool. I always like to use a bigger counter sprocket and a smaller axle sprocket but that's just me.

example:
40/14=2.85
43/15=2.866

The higher the number the higher your ratio will be this will help with small tweaks and tuning on your gearing I know it helps me out greatly when I go to the track I take about 10 different sprockets so that could be a pile of combinations.