What you want to use is a LoD calculation.
First, create a field (I called it [AvgBudgetPerActual]) that calculates the average [Budget] per [Actual] value.
Since Oct,Nov, and Dec have NULL [Actual] values, the [AvgBudgetPerActual] field will be the Average of their [Budget] for these months (1,333).
Now, you want to combine your [Actual] field with [AvgBudgetPerActual] in a new field (I called this [Actual2], where it takes on the value of [AvgBudgetPerActual] when [Actual] is NULL.
if isnull([Actual]) THEN
[AvgBudgetPerActual] ELSE [Actual]
Hope that helps!
CAL_PROB_SF.twbx 54.3 KB