It's not that difficult to measure. Put a piece of masking tape on the back above the shelf at the widest point of the limb. Do the same below the shelf. measure the width of the limbs and mark the center on the masking tape pieces. I then lay a flexible ruler across the center marks, bending it over the curve of the front of the grip. Then, just measure from the edge of the ruler to the side plate. A second pair of hands does make this easier.
Sounds terribly complicated now that I read this, but believe me, it isn't.