If you go to the 3 river calculator the specs on the bow side will ask for that measure. You need to put in +.125 when using the generic R/D bow. It will reduce the required spine. Now, if your release is strong, through trial and error with known spined arrows you will find your personal quotient number at the bottom. Mine is 9 to 11 depending on the bow. Also, a Robertson fires a fairly hard arrow, perhaps more than the generic catagory, that will up the number as well. Personally, I am guessing that variables with full length carbon or for that matter full length wood arrows can toss much of the chart data away. For standard length wood and aluminum arrows, one inch or less past the back of the bow bop, if I can watch the person shoot and get an idea how hard the bow shoots and the individuals release is, I find the chart is very accurate.