How to use @SHIFT(@CurrMBR in a crossdim

I am trying to write a calc in calculation manager which will return a different value based on 3 different situations depending on the Period (i.e. "Jan") in the IF statement. In the example below, for the second situation (Period is Feb:Nov), the value for account "50001" should equal the value of account "Total_Billed" for the prior month. The calc is not working. I think the issue is in the syntax of the cross dim. Any help would be appreciated. Thanks.

My calc looks like this:

FIX (@RELATIVE("YearTotal",0),"Budget")
"50001"
(
IF(@ISMBR(Jan))
"50001"="Fees_Billed"->"BegBalance"*0.5;
ELSEIF(@ISMBR(Feb:Nov))