Hi Silpa,
You can try to use something like COUNT(PERIODSTODATE.... Just test and compare report speed.
IIF may be not so bad:
IIF([TIME].CurrentMember.Properties("MONTHNUM")="1",[ACCOUNT].[INV],IIF([TIME].CurrentMember.Properties("MONTHNUM")="2",[ACCOUNT].[INV]/2,...
B.R. Vadim