What kind of hickory? And as Kevin implied, what’s the beginning moisture content? And is the drying and turning environment climate controlled? The Wood Database gives T/R shrinkage rates of from 1.4 to 1.7 (or 1.8 if you include Pecan). But remember, these are averages and there are other factors plus variations between individual trees within a given species.
I haven’t rough turned bowls since at about 15 years ago so my experience is “old”, if that matters, but when I did twice-turn I used the numbers to calculate the minimum thickness then added a healthy fudge factor.
None of the bowls I turned from those failed to have enough thickness. (I finished-turned a few from my 15-year-old stash just a few months ago) I personally would rather turn green extra thick to be on the safe side, seal wisely, and just be patient about the drying. Better to be way too thick than a little bit too thin - similar to what I always told my kids: better to be a whole lot early than a little bit late. Those I know who twice-turn a lot seem to have developed a good “sense” of what is required for various situations. (Some of that probably learned the hard way.)
I most often twice-turned black cherry which has a T/R ratio closer to 2.
As for long drying time, unless you build something like a fridge/freezer drying chamber, that’s just the way it is. If you don’t have one, maybe get a good moisture meter. A pin-type will work on non-flat surfaces.
Have you considered skipping the drying and finish turn while green and let the piece warp?
These days I only turn dry wood and things are more predictable. Life is good.
JKJ
