If you are having troubles with the minimum length, you could also carry a small continuous loop that would replace a whoopie when needed. Just girth hitch it to the hammock. Say you make a 6" long loop, this would reduce your minimum length by 18" and you will still have adjustability on the other end with the whoopie.
Or if you make it as a soft shackle, it can have multiple uses, like a strap extender, a spot to hang your gear, etc. They weigh but a bit and take up very little room.
My test trees are very close and having a loop on one end instead of the whoopie allows for a good hang angle.