>>2501
The core idea of sight size is to just comparing your drawing/painting 1:1 with the subject. Any use of plumb lines, knitting needles, mirrors or whatever are just an extension of that ability.
Using the plumb line to measure is a little more difficult than just using a knitting needle/charcoal/pencil to check the distances between points. With the plumb line, the shakiness comes from both of your hands trying to stay still while if you use your charcoal/pencil, you can hold one hand out, reducing the shakiness by half and you can even use your other hand to steady the hand holding the tool.
There is a tendency for new students to overmeasure and not rely on what their eye tells them. This often stiffens a drawing and turns it into a mechanical copying process rather than a drawing. On an average 3 hour life drawing I tend to just measure the greatest width and a few vertical landmarks. The rest of my accuracy comes from comparing the drawing with the subject by eye, focusing on negative shapes, comparing angles and the triangulation method. So the way I work is a hybrid of sight size and tools derived from the comparative method. Pushing the envelope shape before getting too far into the drawing is really worthwhile if you're struggling with sight size.
The great thing about sight size is that it is so brutal. You can compare your drawing/painting to the subject 1:1 and by doing so, you can force yourself to be even more accurate and even harder on your drawing. The comparative method doesn't really do this and that extreme accuracy that is so hard to train becomes even more difficult. The good side of the comparative method is that it is so versatile, so it's worth also training with it, or at least some of the tools derived from it.
Imaginary plumb lines are great. Just make sure you connect the top and bottom of the drawing so it doesn't skew off in some weird direction. It has happened to me a few times when not having a clear plumb line. Just connect the clavicle or some landmark of the head to the standing foot and you should be good :)
Anyway, keep working at it and post your results!