From Harold Speed - The Practice and Science of Drawing
So, I've been messing around using dividers, taut thread and a measuring rod as measuring tools to help me "see" line lengths and angles. We've all used grids to help us reproduce or scale an original - and a grid doesn't have to be regular - its lines can just pass through salient points on the original - and that, in fact, is how, I believe, you create a "visual" grid in your mind - by identifying salient points that align in one way or another - usually vertically or horizontally - (landmarks) and measuring the distance and angles between them (with a tool - initially - or by eye). This had better pay off - it's time-consuming. Line! Just when I'd started to think in terms of masses!