I feel it may be a moot point however, we know it's at least 30' x 30' and it could be as small to fit into the palm of your hand. Who knows how big it could have been.
However, I believe this number came from the ratio of the block sizes as it tappers up? Please note the fact I was a solid 2.0 gpa student, it took me 8 calendar years and over 200 credits earned to gain a 4 year bachelors degree in Communications, which required about 115 credits, lol. I'm telling you this because you need to entertain all references as credible to begin the journey, looking at my college transcripts you'd assume I was an athlete, which is true, lol. Yeah, bible schmible torah borah, but there's some seriously deep rooted facts (and similarities) that have managed to stick around despite the influence of man, across the globe and throughout time.
Anyways, I'm trying to say I could give you my mathematical estimate, but I'd rather say "you can fit another block or two on there before you need a point.
Furthermore, the entire structure had an outer layer so where'd that go, maybe there's a trail of cookie crumbs we can follow. I'd assume it was taken down and used for other buildings around Cairo, but if there's a township here and a village there which may contain the same materials we could end up finding the capstone. Just a thought, because I entertain them all:).