If you make a bendy handle bow, with a perfect arc of circle tiller, string-tip angle is 90degrees at a draw length 1/2 of the string length.
For for a 39" bow, 36" string, you should be able to get 18" before it stacks horribly. But this all obviously has nothing to do with whether the wood will survive.
I think 16" should be fine though. it's equivalent to 32" draw on a 78" ELB.