It took me a while to figure out skid patch calculation: turn your gear ratio into a fraction and then divide the top by the bottom. If the result is an integer, that's the number of skid patches. If it's fractional, than the number of skid patches is the bottom number. So in the case of a 48 tooth chainring and a 16 tooth cog you've got 48/16 = 3, so 3 skid patches. In the case of a 48 tooth chainring and a 17 tooth cog you've got 48/17 = 2.82, so 17 skid patches.