I played around with this app last night with several wheels, measuring different sounding spokes on the same wheel, and trying different input values for length and diameter on the same spoke. First, the output tension values were quite consistent with the same spoke and input values - typically repeatable within 1 kgf. And it easily indicated a different tension value for spokes that sounded slightly different, as it should, but repeatably showed the same tension for same-sounding spokes. Changing length by 2-3mm didn't make much difference, but the wrong spoke diameter changed the indicated force by 5-10 kgf.

At one point I was doing this as my wife was using electric toothbrush in a nearby bathroom - that changed the base level quite a lot, and added some error. So the directions to use in a quiet place are appropriate. It didn't seem to matter much with my wheels if I intentionally damped the other crossing spoke.

I don't have a tension gauge to compare with this (and with this app, I'm not likely to get one), but the outputs it showed seemed quite reasonable compared to the recommended tension values I've read. BTW, all the wheels that I was checking have proven to be extremely reliable, and I don't have any "bad wheels" to try. But it did show the quite high DS tension I expected on my most recently built rear wheel that has about 300 trouble and tweak free miles.