subtilities of shapes and shadows

If drawing is all about putting the right information in the right place, which is far more difficult than it sounds, I struggle sometimes to find ways of training my eyes to see the information accurately.
I spend a lot of time trying to see the relationships between shapes and shadows,  four or five hours minimum for each drawing. 
All the time, I keep forcing myself to see more accurately the subtilities of the forms, light and dark. I spend hours moving my eyes from one point to another on the cast/model and then moving my eyes between the same points on the drawing, making sure that the relationships between the points match.
Only once I decide the proportions look right, do I start to draw by the larger shapes, followed by the smaller ones. 
I must admit that, where possible, I get a third person to check my proportions  before I start working on the light and shadows!