I have made similar test apparatuses for other purposes, so I will answer just to keep Jim's workload low, to keep him fresh to provide a comprehensive answer to my questions : -)
First, one positions the apparatus on the guitar, unloaded. Then, one zeroes out the dial indicator. Next, you load the device with the reference weight, and then record the new reading on the dial indicator. The difference in thousandth's of an inch is the deflection under load.
In general, as long as you always use the same weight, the greater the deflection under load, the softer the top. Very stiff tops will show little deflection, and soft tops will show larger deflections. The measurement take place via the indicator, but the "data" it provides is relative, not absolute. It is only useful for comparisons. One might build a similar looking device to measure the relative stiffness of trampolines or fishing rods for that matter, and they work, provided you only use them for their specialized purpose.
Now at the risk of complicating the explanation -yet for the sake of completeness- it is possible to calculate the absolute stiffness or rather the Modulus of Elasticity (or MOE) of materials with such a device, provided several conditions are strictly met, e.g., measuring uniform beams of known dimension, with a reference mass, and a known span of supports, and then calculating the various values using a specific mathematical equation that can be found in the book Understanding Wood, by Bruce Hoadley. I have built devices just for this special purpose, but again, Jim's device is for simple comparison of one guitar top to another. As Kent noted, it will not necessarily reflect strain under torque, which is a different kind of stress than simple top loading.
The very reason I asked the question I did, is so I can perhaps use his empirical observations rather than having to build a differently calibrated device and having to sample many guitars before having some idea of what to look for. As long as I build a device with the same span and weight, I should get similar results. The weight of the board or the spring in the dial indicator is negligible, since the dial indicator gets zeroed out with these things taken into consideration before the weight is added.
In reality, it would make sense for builders to settle on a reference span and weight, so that everyone can compare notes. I would submit that Jim's device is as good as any for a universal reference, once he tells us what he used : -)
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