Originally Posted by
robk
Basically, if you think of the spoiler as a right (90*) triangle, the chord would be the size of the angled side, and the height would be the vertical side. Obviously it's not strictly a right triangle, since I have no idea how those numbers work out, but you get the idea.
An easier way to think of the chord is the same definition used in the aircraft world; the distance from the leading edge to the trailing edge. Doesn't matter what the angle is when you measure it, just be sure it doesn't exceed 20mm from the front edge to the back edge of the spoiler and you should be good to go.