Wanted: 14T Track Cog 1/8"

This seems like a good point to ask this. Short of fitting it to the bike, how will I know if I can use a 14th without reducing my chain size. Is there an algorithm I can use or some measurement or other. I can just fit an 18t with about 5mm to spare on campag drops.
on my black bike i go from 45:18 to 45:14 without drama