I find a single point on the reference image and then locate two points that are near that first point and on the same form, so like if I was drawing a face I could pick the corner of an eyebrow, the corner of the eye, and another eyebrow corner, and then I draw a triangle between the points. I can then use any of those 3 points to draw other triangles to other points and then shit sort of works itself out.
Also using relative measuring with your eye can work but you have to train it. Checking how long X is compared to why or checking how far away from X Z starts and shit.
Also this didn't deserve it's own thread. you should have posted in /sqt/