French Bass Brace advice

[Adrian Levi]

The ribs are now as clean as I'm going to get them without doing more damage to the damage.It seems as if the bass was about 50% wood and 50% epoxy.
The one lower bout is the only one left to reinforce . The other lower bout with the repairs was less than 1 mm in places and still needs the back edging to be replaced.
Now that the two pieces of back are flattened and ready to plane for an insert I'm not quite sure how to figure the exact size that the insert should be.
The back has shrunk a fair amount and to compensate the ribs were force epoxied in. The freed rib structure now spans about 20 mm more in its relaxed state.
Is it a matter now of lying the rib structure on top of the back to measure the space needed for insert ?
Or is it better to measure the back against the top for size ....?