In the misty recesses of my memory I seem to recall seeing that if you deined the calculation twice in the same query it had to execute the query twice, and that aliasing it and referring to the alias was better. But now I'm thinking that involved HAVING clauses.

[Edit]: And BTW I was reading the code closely, the explanation not so much. So I missed that. Though since you didn't notice until you went back a third time, I'll say it wasn't that obvious anyway. :-P