Hugo,

I really like this kind of question 🙂 - it helps keep my brain from atrophying (I hope :-D). More please!

I plumped for the error message, because I reckoned an index with 200000 leaf pages would need about 3GB for the rebuild operation, so saw no need to calculate the size of the non-leaf part of the index. Was surprised to be told it was wrong, but not much bothered: mistakes happen.

edit: I didn't even notice the switch from 200000 to 20000 in the explanation (which apart from that was an excellent description of how to calculate the final size); can't imagine it would confuse anyone who did notice it.