Hi Norm
I've repeated the measurement you asked for against my S2 (which to the best of my belief has the original rear panel and that appears undamaged and unwarped) and I make the measurement 10mm to your 17mm:
As you've already mentioned ... it might be worth getting things checked against other Fulvias at the AGM then you can be more confident ... and in case my panel isn't the epitome of a good reference model!
Regards,
Scott.