Using Aggregate Functions with a Guage

Hey all, 

Currently I have fields for:

Computer Name

Last Check to the Server.

I also have calculated fields to determine whether these computers are considered "active" or "inactive".  What I want to do is have a guage that displays the percentage of "active" machines.  My current code is:

=IIF(Fields!InactiveCheck.Value >= 90, 0,(1/(COUNT(Fields!Computer_Name.Value)))

Thus each Active Computer will give me (1/total computers) and I can use the guage SUM function.

The problem is that I cannot use the "Count" or "Sum" function within my calculated field and the total number of computers changes constantly.   Any Ideas?

Thanks,

Nick