Slope or (inch rise) is the angle that the roof makes with a horizontal plane. It is determined by finding the quotient of the rise divided by the run.

In any case total rise / total run = quotient . Using the inverse or ArcTangent function (tan^-1 ) on the quotient you find the angle of the slope. Inch rise is the actual inches you travel up vertically for every 12 inch you travel horizontal.

In any case it's (total rise*(12)) / (total run) = inch rise

The inch rise is what you'd set the stop on the tongue of your square at. 12 would be what you'd set the stop on the body at.

This is a most useful method when you know the height of the ridge and the run of a section of roof, but do not know the pitch.