I don't think you need 50% extra for each additional rabbit, because they share a lot of that space. If you have one rabbit, you can't just have half the usual recommended space. I vaguely remember seeing something say that you need 20 or 30% more footage for each additional rabbit, but I'm not sure.
I have 30ft of hutch space and 38ft of run space for three rabbits, but they do get a lot of free ranging time in the garden or the house (they're very active though and may be a bit wild). I wouldn't go any smaller unless it was only temporary. You do have to make sure the bedroom/hiding space is big enough to accommodate all three comfortably, or that you have several hiding spaces.
But I agree, as big as it can be.