How I determine the thickness for a guitar back 

Thank you very much. I'm glad to hear that others are doing this as well.

Yes, there can be a good deal of difference between samples of wood of the same species, which is why I test. I standardize on a smaller piece because I build more than one body shape, so standardizing on a full back would not be helpful. In point of fact, I used to measure deflection in both directions and on the back itself. I found it awkward and unreliable with way too many variables to keep track of and account for. Whenever possible I try to keep it simple and effective.

@@chuckmorrison so is the piece you test from the larger piece you intend to build from? I often don't have enough material to sacrifice a small sample for testing. I like the test setup, I guess I'm just wondering how to put it to use for each instrument I am building. Sorry if I'm being daft, I'm always looking for ways to make these types of assessments and I don't want to miss the chance to learn something that seems like it would be very helpful just because I'm missing something here

Yes, the piece I test is usually from the area right next to the upper bout. I usually resaw my backs and have the spare area to test. Most presawn back wood also has enough to test in this area.

yes, it's from that day. I used the unedited version.

Yes. It's not rocket science, but it beats just thicknessing all woods the same regardless of density and/or stiffness.

@@chuckmorrison, as an engineer, I appreciate your approach. How do you handle the thickness of a soundboard/top?

Ah, It's a bit more complicated. I use 5 different species of woods for tops AND double top variants of each. Sitka spruce (rarely as solid, very heavy and stiff), Nootka cypress (rarely as solid, very flexible across the grain), Western red cedar, European spruce, Englemann spruce (similar to Euro spruce's stiffness & weight but softer). Of course wood varies within each species. For solid tops, I start out the same way measuring the longitudinal stiffness of a piece (all standardized dimensions except for thickness) of the top (from cutoff scrap). I compare it against a spruce/nomex/spruce or cedar/nomex/cedar standard stiffness and weight. I thickness the piece until it approaches the sample's stiffness('s). I determine how stiff the wood is across the grain and weigh the soundboard at the tested thickness. At this point I decide if it's a piece I want to use or not. If not, I repeat the process on the next candidate.

I maintain my shop at 40% relative humidity (~68 deg F) all year long. All measurements are done at that level.