[gilleain]

Yes, I've learned some techniques after reproducing a lot of Islamic patterns, which can be done entirely with straightedge and compass (although frankly I often 'cheat' by using measures, grid paper and tracing paper, lol).

Indeed the movement of the pencil can matter a lot, as can how good your compasses are. I've found that a bevelled ruler can be better if turned over so that the pencil rests against the bevel. Otherwise you have to take account of the width of the pencil lead and adjust the ruler position to take account of it.

Certainly small errors in line position, length and (especially) angle can have a huge effect over a large diagram that has a lot of symmetries. It's surprising how noticeable even a small error of 0.5 mm between two lines is that should be colinear.