Recommended moisture content for painting bare wood
I figured it out. 12% is ideal (a few percent less for dry climates, and a few more for wet climates). Never paint on anything with 20% or more.

I've worked on several homes with bad flaking paint. One time I thoroughly scraped and let dry in hot weather for a week, and used high quality primer and paint, only to be scraping flaking paint again in 4 years. 

I've consulted several sources on the problem and gotten mixed advice. Some said that it was inherent in wood siding, but I've seen 15+ year old painted siding in the same weather conditions without a single flake. Someone else said that it was a bad first paint job, and the siding is water logged, and that once the siding is water logged, it's ruined and will have to be repainted every 3-5 years. The most persuasive and logical answer that I found, in my opinion, is that some previous paint job was done with too high a moisture content in the wood, and every subsequent paint job will suffer from a slowly (3-5 yr at a time) peeling problem. The only way to fix it without replacing the siding is to remove all the paint with stripper and/or pressure washing and allow to dry to a proper moisture content before priming.