Assuming you're not dealing with a carbon frame, you're making it overly complicated. Use the string method Sheldon B. describes to verify the centerline, and a scale to verify original dropout width. Then cold set applying the same principle as for breaking a turkey wishbone, meaning the side held farthest out usually gives first.

Lay the frame on the floor and stand on the lower side of the rear triangle, about one third of the way up. Lift the upper one from the end until you feel it give, and check that you've moved it half the distance needed. Repeat as needed then flip over and repeat for the second side. Double check that the centerline is still as before, then use dropout tools to bring the frame ends back to parallel.

Note, that this method is not 100% reliable depending on how different in strength the chainstays are, which is why you need to keep track of the true centerline. In any case you don't need to get overly concerned over stressing the front triangle because these are always much stronger than one side of the rear triangle alone.