Where the rake is the combination of two angles the actual slope of the rake is a transcendent trigonometric function of those two angles. I ran off with the theater to avoid having to deal with algebra; and Trig is Algy's bigger, meaner older brother.
This construction derives the correct angle (true dip perpendicular to the strike line) using only a compass and straightedge. No calculation required.
If the slope angles are given as Run/Rise this is very straightforward (but you will have to measure.) Mark the two unit rises and draw the two angles. If the slope angles are given in degrees or you are dealing with wild numbers (or to fit an as-built); begin with the angles, copied with a bevel gauge or generated from a protractor, and extend them through the perpendicular to obtain AC and BC. To obtain ½ AC or BC use: http://www.mathopenref.com/constbisectline.html or, if you have easy numbers you can measure.