"Use where [10%] > 1000 "
That's what I recall, that you can use aliased names in a Where or Group By clause. But this returns an error in SQL 2008.
Select InvoiceTotal,
InvoiceTotal*(.10) as "10%",
InvoiceTotal+(InvoiceTotal*(.10)) as "Plus 10%"
From Invoices
Where [10%] > 1000
Ken