I'd like to experiment with 9" rear-tapers on POC's, Sitka Spruce, and Douglas Fir 23/32" & 5/16" shafts.
Do I just use the usual "Kelly-type" spine charts for parallel shafts and hope for the best, or are there 'spine-taper adjustment factors' for judging the effects of rear-tapering shafts?
Should I re-spine test the tapered shafts and go by the "new" readings?
You should respine them after tapering, and go by the new reading. You may lose a few pounds on the spine tester, and it's real. The charts for parallels will work.
Tapered shafts really do work better for most folks.