There is really no right or wrong way for calculating wing area. There are various conventions common in the RC airplane industry. The most popular is, of course:

Overall wing span x average wing chord.

Wing span is easy: measure wing tip to tip - that's it! Average chord is more subjective. I personally prefer:

(Root chord + tip chord) / 2

This does take the area between the fuselage into account, but it is slightly different from what Dick described above.. Unless the wing is highly tapered, though, the results are close enough for estimating wing loading. I guess "estimating" is the key word. If we truly are calculating "Wing loading" then we really should calculate the wing panel ONLY. However, most of us are actually more interested in how the airplane behaves aerodynamically. Since the fuselage does act as a lifting body at most angles of attack, increasing the wing area to include the part between the fuse is good enough of an estimate to compensate for the added "lift" due to the fuselage body. Truth is, the added lift really depends as much on the fuselage shape as its width near the wing.

To answer the original question: 31 Oz/sq.ft. is just fine for the typically 72" aerobat. My GP 1/4 Giles 202 is almost exactly that, and my GSP Extra 300 and Yak 54, both 72", look like they will also finish out about the same.