ATAN Trig Function

I need to find an angle , inverse tangent, based on (2) sides, in the Product Configurator.
I can't seem to make the 'ATAN(Expression)' Function work.
Anyone with experience with this particular issue?

Need Mail bonding?
Go to the Yahoo! Mail Q&A for great tips from Yahoo! Answers users.

[Non-text portions of this message have been removed]