The proper way to measure the TT length has never changed. It has always been from the HT/TT intersection point, along a horizontal line, to the center of the seat tube, or the center of the seat post, if the TT slopes. That makes it possible to compare traditional frame to sloping TT with NO error. Manufacturers may call the TT length "effective" or "virtual" but it all means horizontal.

The height at which the measurement is taken will not make much difference, since the TT and HT are either parallel or close to it. A difference of 1-2 degrees over the short distance from the HT/TT intersection point to the of the head tube would make a trivial difference.

What's also being overlooked is the effect of differing STAs. If a STA is one degree steeper on a frame, it increases the reach by about 1cm.