Marianne I have 2 measurements of Walter soundboards, one from Kunsthistorisches Museum in Vienna, with about 15 measuring points, and other just roughly stated in technical sheet (but I'm not anymore sure about which instrument). I can send you the documents if you want.
The first measurement gives suprisingly thin values (about 3mm for bass and 2,5mm for treble) and other simply gives value 1/8bass-5/32-1/8treble (obviously in inches).
I think the values that Richard gave are very logical, somewhere in between these extremes.
Pavel
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