GeneralTotal for accumulative sum: closure and begining for complex period

Hello!

I faced a problem: In my VS2019 SSAS project I use mdx script sum({null:[Time].[Periods].PrevMember}, [Measures].[Account41Plus] - [Measures].[Account41Minus])  to get accumulative sum at the opening and sum({null:[Time].[Periods].CurrentMember}, [Measures].[Account41Plus] - [Measures].[Account41Minus]) to get accumulative sum at the closing of  period. GrandTotal works well in case of “simple” selection of period: one year or one month or one day. In case of ”March + February” or March 1+March 2+ March 3 I see empty GrandTotal for “before” the “complex” period and wrong GrandTotal “after” the “complex” period.

What can I do to get “most feasible” GrandTotal of accumulative sum in my SSAS cube?

Thanks in advance!