Originally Posted by
ejewels
Agreed, I was asking more about where one should measure that angle to... (the top tube or horizon/floor).
Along the line of the effective top tube, IMO. Parallel with the wheel axles.
Not the horizon, because the position relative to the bike affects comfort (more than orientation with gravity), and the bike's motion is always relative to the bike's frame (in line with the wheels basically) so aero is not with respect to the horizon.