Chris,
I always figure 80 square foot = 1 yard. That is the typical calculation for a 4" slab. I use 1"x4" 's as form boards. It is always better to order .5 yards extra due to imperfections in your base, and any irregularities in your framing being square. I figure .5 yard is $50, so it is money well spent. Just browsing over your #'s, it looks ballpark, so I'm not certain what your issue was. Did you place a moisuture barrier down first in your forms? It certainly looks as though you did it as anyone else would have.
CBK,
I did not put down a moisture barrier. Just poured over the packed sand. I really thought ordering 20% extra would have been enough. The other problem, is the UCart concrete only comes in trailers up to 1.25 cu yd.. I could have ordered a truck, but then I would have about a 3 or 4 yd min. The frustrating part of this, is somewhere someone is off... either my math, my prep work, or the concrete guy. If I had a 10 yard pour figured with these same calculations, and I figured an extra .5 yd, I'd still come up 2.5 yds short... that's how far off this job was. 30% best I can figure.
