Drawing Heads

Recently the problem of drawing heads from memory has surfaced again for me. When dealing with drawing problems I try to find a logical and structural solution to them. So I know for sure I've grasped something, so I know it's not just some phantom in my hand after hours of trying that's doing the drawing. Logically, it would make sense to start with a basic form, then keep building on that of that. That way things can be kept track of, using rules of perspective. This is in a way mathematical. Realistically speaking, it is almost humanly impossible to develop the entire form a head so you have mathematical assurance of the entire form's accuracy. That would mean you have to have total accuracy along each step of building the form. And that is why one needs to draw from life. The impressions of natural variations acquired such way fill in the blanks where geometrical structures fail, and with much higher accuracy and efficiency.