Originally Posted by
repechage
I would look at it this way, if everything else makes the drawing, and the path of least resistance means fudging the level of the top tube, why fight it?
The drawings of bikes with horizontal top tubes seem to use the TT as a datum and mark the HTA and STA angles from the top tube, thus it is easiest to measure the same way. I believe that tolerance stack, measurement accuracy and inconsistent changes made to the lengths/angles of tubes of different frame sizes contribute to +/- 1 degree of accuracy.