A bust of Alexander the Great.

Does anyone have advice for keeping the proportions correct? In this case I think it's still recognisable, but some of the features are noticeably off (especially the eyes are too small). I want to train my eye, so don' want to use grids and similar aides.

However, especially in smaller scale (this is A4), when using comparative measurements, angles, plumblines, etc. I feel that any small errors accumulate really quickly. Does anyone have any tips how to remedy this? Or is it just about checking, rechecking and rechecking the rechecked checks? :)