Originally Posted by
Metacortex
In that case either the factory drawing geometry is wrong (unlikely) or a combination of tolerance stack, geometry variations due to frame size differences (geometry varies slightly in sizes other than on the official drawing) and finally measurement accuracy all contributed to your 0.7° measurement. I would still consider that a horizontal top tube even if it was possibly built or measured slightly different from what was intended.
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?
Level top tubes are not gospel, I do not know any builder who uses that tube as a datum, so no problem.
Unless you want to use it as a datum later on for reverse engineering... Then you get errors.
I really like using the wheel axle centers, makes it pretty easy to measure off bottom bracket drop too.
That is more interesting than the accuracy of a level top tube.