MDX help for a Cumulative % over time for analytic grid report

I have a source with two facts as the table below:

I am using performancepoint to create a KPI and an analytic chart over time.  The KPI is the percentage of successful events, so in Jan this is 80%, in Feb it is 80% and in Mar it is 50%.  For the three months in total it is (21/30) 70%.

I have a time dimension which is configured for time intelligence and I have worked out the KPI fine for a scorecard using time intelligence filter so I can look at individual months and year to ate etc and the percentage work out the correct answer.

What I want is an associated chart which shows a graph of the % over the three months giving the result of 80%, 80% then 70%

So I have the following MDX query:

WITH Member [No of events] AS [Measures].[No of events]
Member [No of successful events] AS [Measures].[No of successful events]
SELECT
HIERARCHIZE( {
[Dim Date].[Fiscal Year Month].&[FY2014-01],
[Dim Date].[Fiscal Year Month].&[FY2014-02],
[Dim Date].[Fiscal Year Month].&[FY2014-03]

} ) * { [Dim Date].[Month Name].[Month Name].ALLMEMBERS }
ON COLUMNS,

{
[Measures].[WaterCommittmentsDeliveredOnTimeActual],
[Measures].[WaterCommittmentsDeliveredOnTimeForecast],
[No of events],
[No of successful events]
}
ON ROWS

FROM [persp_cube]

How can I define the [No of events] member as a cumulative member for each month so I can work out the accurate percentage?

Thanks,

Dan