Have calculated the MAX over the running total:

WITH MEMBER [Measures].[RunningTotal] AS

aggregate(

PeriodsToDate([Calendar].[Date Hierarchy].[All].level,[Calendar].[Date Hierarchy].currentmember

),[Measures].[Amt])

MEMBER [Measures].[MaxAmt] AS

MAX(EXISTING [Calendar].[Date Hierarchy].[Date].members, [Measures].[RunningTotal])

SELECT

{[Measures].[Amt], [Measures].[MaxAmt]} ON 0,

[Account].[AccountId].&[Acc1] on 1

FROM

[MyCube];