not sure of the terms you are using.

lets say you have a ridge roof (has a peak in the center) and the total span is 6 feet. and you chose a 3/12 pitch. this means that the roof will *rise* 3 feet for every 12 feet of *run*.

since a single plane of the roof is only 3' of run (12'/4), the rise, or peak will be 9 inches (3'/4) above the top sill plate.

using pyth theo, (inches) side one squared + side two squared = square root of hypotenuse (rafter length). or 36^2 + 9^2 = 37.1".

just remember that this dimension would be a line, measured on the bottom of the rafter as it sits on the top sill plate and up against the ridge board. then I usually draw out (subtract) half the thickness of the ridge board before you make the cut. angle calculations are another story.