You can also use some silicone attached to the glass before spraying it. That will give it somehting that it can adhere to.
Ive never used it in a reef tank, but Ive done all kinds of stuff with it in vivariums like build rivers, water fallls, planters, and cliffs.
I think adding rocks to it will give it a better effect. I also think it can be used to hide cords and outflows if planned proerly.
One other idea Ive had is to silicone rubble and CC or sand to it after it cures. This would make it look like the face of a drop off. With your rock work tied into it, it could have a great amount of depth and detial.
Here are a link to a vivarium I used it in covered with susbtrate.
http://img.photobucket.com/albums/v618/freekfornature/Tropical Paradise/2005-08-30009.jpg
That water fall on the right and the river were molded out of foam and covered with stone. The sides and back of the tank were also sprayed, then I modled in the planters and stuck wood in there for added effect. It was then all coverd with silicone and peat moss. All the cord and tubes for the pumps are embedded in the foam.
Oh and to answer the orginal question, I sprayed 4 cans in to the 29 gallons sides and back which is a lot smaller than your area