If you followed my work in the past, you will know that I will layout my painting or the piece I am about to paint in photoshop.
I create the actual size on the computer.
I then print off section of the piece and it is these print outs that I use as reference to transfer my image to canvas.

So here you see a print out of Jarome's head.
I also have placed a couple of marks that I will use for starting points. I call these reference points.
Which basically keeps me on track with the composition layout that I created in the computer.

Throughout the piece I will literally do measurements from the print outs so that I can capture the detail of the portrait.
If you think of it in this way...
I measure-draw,draw,draw,draw- measure,measure-draw,draw,draw,-measure-draw,draw,draw and so on.
Until the drawing is complete.
This method gives me the most accurate drawing as possible.