Sorry, I don't know the answer - what I usually do is this...
My decal printer prints exactly what come off my regular laser printer in terms of size. So I use Adobe Illustrator, tweak the sizes and cut the lettering out on paper and place it on the model until I get the size that looks right.
It's a pain, but eventually you get sizes that look right.
Another thing you could do - if you have dead-on, side view, 90 degree photos of the cars is to measure a known distance in the photo as a reference and from there you can get pretty accurate measurements anywhere.
