user defined aggregate

I utilized syntax:

//if  [Period] = str(DATEADD('month',6,TODAY()))  then [Var.val.in rep.cur.] End

if  [Period] = "8" and [Sheet] = "CY YTD Data" then [Var.val.in rep.cur.] End

To generate Actuals and a similar syntax to calculate Forecast.

I am now attempting to sum Forecast - Actuals but I encounter the error 'the calculation can't be applied to a user defined aggregate'

I've attempted formulas:

SUM([Forecast]-[Actuals])

SUM(INT([Forecast]))-([Actuals])

But no luck,

Any thoughts....