I've only recently started using MathCAD for this kind of thing. It's a handy bit of kit because all your equations remain live so when it comes to knocking out revisions it halves the time because I only have to change the initial bit of math and it updates all related equations for me. Before I was using a mix of Word, Excel and scanned hand calcs which works fine but it's not as convenient.
The only real gripe I have is that it is not a very good word processor.