I think it's better to think in terms of useable floor space. Where can the birds stand and move around in. In an A frame there is dead space at the edges. This isn't really usable in terms of being space the birds can comfortably occupy. For the purpose of calculating square footage, I would only count area that had at least a foot of headroom above it. For most of the A frames I've seen that would mean losing a foot on the angled sides. So a 5x10 A frame would really have closer to 3x10 usable space. Others might look at it differently though.