>>7759777
i'll try my best to explain it.
an object in perspective is going to be distorted so to get exact measurements on its surface, you simply draw your measurements out on a line where there's is no distortion, which is going to be the line that is parallel to the two vanishing points of that surface
and this holds true no matter what the size of your measure line is. you take the end of your measuring line, match it to the corner of the surface you projecting to, and then run it to your special vanishing point, and then you can project the rest of your measurements to that surface from that special vanishing point.

and here's a freebie, if you have a set of horizontal lines on a surface but want to make them vertical, or vice versa, just run a diagonal line through the surface, the points where it intersects those lines are where you will run your perpendicular lines with the same measurements.