I understand your desire to calculate this with some simple geometry but there are other things at work with any cantilever type brake. The first and foremost is the flex. Cantilevers (Or V-Type) flex the fork outwards (Someone once mentioned that you should put your hands around a canti-fork and squeeze the brakes, you will feel them flex).

Also your diagram should be drawing Y to the location where the brake pad is connected to the pivot arm, your diagram assumes that the braking force will move perfectly perpendicular to the ending of point 'Y' where in reality it moves at an angle.

Long and short is: V Brakes have more flex in them which will change your calculation. Don't underestimate the effect of flex.