There is no convenient method to calculate room modes for non-rectangular spaces. It can be done, but requires sophisticated software as opposed to (relatively) simple calculators.
What should be avoided is to get sucked in by the "non-rectangular rooms don't have mode problems" myth. I see this one from time to time on the (oh-so-reliable) Internet. All rooms have modes; they're just much more difficult to predict in non-rectangular spaces.
What non-rectangular rooms are very good for is minimizing problems with flutter echo. Fewer parallel surfaces help enormously with this. The sloping ceiling—provided the low part is in the front (screen) and the high part is towards the back (seating)—is also a boon.
Whether or not some approximations can be made about your particular space will depend on the exact layout, which I assume you'll be providing to us in that new thread, yes?