I've been having a go at measuring the downbearing in my upright prior to removing the strings. It's a 1937 Kemble and (to the best of my knowledge) has never had soundboard work done on it since it was made.
Before I go into details of what I've measured, my first question is whether it's typical for an instrument of this age and my second question is should I attempt to adjust it by putting on thinner understring felt.
I don't have a proper gauge, so I used the method described in Reblitz, using dental tape stretched across between the top and bottom fixing points. However I found it extremely difficult to measure with any accuracy.
All I could conclude is that at the bass bridge, there is somewhere between zero and slightly negative downbearing, increasing to a slight positive downbearing in the treble.
The soundboard itself appears in good condition, with no cracks that I can see, and the ribs are solidly attached for their full length - the ribs are vertical on this piano.
I can measure a slight crown by putting a 2 foot steel rule between the ribs and measuring the gap at the centre with feeler gauges. A similar crown (obviously in the opposite direction) is measureable on the front surface of the soundboard (but this is only accessible at the far left hand side).
The gap measured between each pair of ribs at the rear varies between about .040" and .060".