Using existing expressions in new expressions

[#1]

I have a calculation called Paid FTES.  It build off of 4 variables and works correctly.  I want to reuse the expression as shown below:

=sum({<FORECAST_LEVEL_DESC={'TOTAL FTE'}>} "Paid FTEs")

If I reference Paid FTEs like this: ="Paid FTEs", I get the result.  However, if I wrap it in a sum so that I can push the value down to the children of Total FTE, it comes back blank.