There are tons of good online calculators for this stuff, but this one does it Death by Bunjie style! LOL. In fact, this one comes pre-loaded with the specs of the Original Bunjie, how about that?
You're going to need the weight (in grains) of your arrow, and how fast that arrow is! We're talking total grain weight (arrow plus field point or broadhead) and an accurate speed. Scales and chronographs are best here, but if you don't know for sure, try using the manufacturer's speeds and weights. For excample, for Excalibur, try 350 grains. For Ravin, try 400 grains. Then use the manufacturer's "rated" speed for your crossbow.
This might not be 100% but it's close and it will give you an idea what we're doing here. Again, this is just a fun tool for discussion.
Play around with different arrow weights. For example, what happens if you change your 100 grain field point out for a 150 grainer? The added weight will likely slow your crossbow down between 12 and 15 feet per second-but your KE and MO numbers will likely go up! Fun stuff!