If @PREVSIBLING returns null, have it return the member being evaluated instead. Is this possible?

Summary:

Is it possible to return the current member being evaluated in @PREVSIBLING if the formula evaluates to null (the member being evaluated does not have a previous sibling)?

Content (required):

I am trying to write a calculation script that calculates a quarter-to-date true-up. It is simple to do this when the period is month 2 or 3 of the quarter, but for month 1, I need to be able to just calculate directly on the current month. See an example below:

"BU1" =

(@SUMRANGE("BU4"->AC1"->"TP1",&Act_QtrMth1:&Act_CrMth) * "Alloc_Pct") -

@SUMRANGE("BU1"->"AC_BU1_Alloc"->"TP1",&Act_QtrMth1:@MEMBER(@PREVSIBLING(&Act_CrMth))