GingerK, atm I've got the corners rounded w/the radius starting at the corner of border #3 (the gray bits around that border are supposed to be empty space, sorry). My CG software claims that would put the corner drop at about 29", which still may be too much. Side drop is 25", measured from the seam on the mattress to about 1" above the floor - my mattress top measures 80" from seam to seam. I imagine the extra couple are due to the cushioning on the mattress.
I am going to test that drape with a bit of muslin, but since I'm doing that last border at the very end, I can always fudge it if I feel the need.