Approx. 10.7 cubic feet of fill total. Using Mel's recipe that would be about 3.6 cubic feet each of vermiculite, peat moss and compost.

You already have all the peat and vermiculite you need. Now you just need to figure out a weight to volume conversion for the compost you already have. That's the tricky part - I would just suggest eyeballing or measuring the bag dimensions.

10.56 cubic feet of mix. You almost have enough to fill it now...I would mix in what you've got, then water it really good. Most likely, it will shrink an inch or so. Then, add some more compost, and blend in with your hands.

EG

Supposedly, a 40 lb bag of manure should equal 0.75 cubic feet. I used that rule of thumb when filling my beds, but it didn't seem to hold up. I needed quite a few more bags than I calculated.