[Garth Herrick]

Mike, That's an excellent technique.

Here is another Photoshop variation I use:

First scale the reference image file in Photoshop to the same dimensions as your painting (in whatever units of measurement you prefer). In the top menu under View, select to view Rulers,which gives you a ruler scale on the left and top of your image window. Wherever you place the cursor you will see its measured position in the scales to the edge.

Say you need to know the position of the inside corner of the right eye. The cursor, placed over that point indicates perhaps 10.75 inches down from the top, and 16.25 inches horizontal from the left side. All you need to do is copy those measurements with a tape measure on the canvas to mark the point down from the top and from the left, and there's the precise spot for that inside corner of the right eye.

This is the same principal as Mike's photo scaling above, except that you always have the convenience of a 1:1 scale ratio.