[Brenda Ellis]

Hey, I do this too!
But I use the inexact, impatient person's method. (No reflection on you, of course, Richard.)
I take a photo of my painting with my digital camera, download it to my computer, take it into photoshop along with the reference photo, make the lengths of the heads the same (since this is one measurement I always measure right and keep) and then drag the photo over to the painting or vice versa and then change the opacity of the top layer so I can see where the discrepancies are. Oh, this also requires using the same size file . Doesn't matter if they are not exactly in the same place because you can drag the top layer around til you are right over where you want to be.
I use this method when I am at a total loss and can't figure out where I've gone wrong. Usually if I give myself some time, I can figure it out on my own.
I think the thing I like about my method is that I still get to challenge myself to make the corrections with my own eye.