I don't know, it's just what I read, most growers consistently got right around 45 g sq/ft. Then when I got my highest yield last time it worked out to 45, so I have reason to believe it. And the way I do it is to take most upper leaves off so it actually does become 3D, even bottom buds get thick. I got a lot less when I just left all the leaves on, got 80 grams a recent time I did it that way. I did put a stronger fan in the chamber recently too though, so that may be what was keeping yields down before. Weird though because the plants grew vigorously, just didn't yield much. I was using PC fans with carbon filter pad over them and it was just inadequate. Changed to bathroom fans and apparently it helped.
BTW, the reason why it makes sense to remove upper leaves in indoor grows is that unlike sunlight, lamp light can't penetrate far. Outdoors, maximum leaves might equal maximum yield but indoors probably not. Just equals maybe 1' of thick buds and 2' of useless fluff.
I only do the defoliation as required though, not just wholesale hacking. I only remove leaves that are causing significant shading of bud sites, which is actually most upper leaf. I don't remove bottom leaf at all, unless they're dying. As long as you have a fairly solid few layers of leaf on the bottom all the light will be used anyway. Just that now those bottom leaves serve a useful purpose rather than just dying off as they usually do. I also don't lollipop, because I want those lower buds to fatten up, maybe a few really thin very bottom ones need removal but not many. The idea is to have a healthy bottom area with a mixture of leaves and buds but everything above that is pretty much all flowers. It's only on the very last week that I really clean up the upper colas, basically manicuring them while they're still alive. Saves a lot of work later too. When you cut a leaf off you have to get the whole leaf stem right at the stalk. If you leave leaf stems on they will mold after they die and partially dry and it will then spread.