20 / 15 is a 4/3 ratio
if the hammer spring is 18 then , take 18 and divide it by 4 = 4.5
take 4.5 X 3 = 13.5 not 17 ... at least that was how we calculated ratio's in 1967 high school old math class...it may have changed , so much has .
Gary

__________________
Certified Cajun
NRA Member