Row Identifier within Power BI for Desktop (Circular dependencies with the DAX CALCULATE function)

Hi WillT I did try the row ID trick but no luck for my scenario. 

Although I've gotten by the issue by defining a measure instead of a calculated column, I thought I'd spell it all out here in case you have any insight.

I am trying to add a calculated column which is in the form Column = CALCULATE ( ....) which I understand from Alberto's post:

"The problem is that any calculated column containing CALCULATE (or a call to any measure, which adds an automatic CALCULATE) creates a dependency from all of the columns of the table."

As a precaustion I deleted all other calculated columns on that table but it didn't help. 

The reason I was trying to set it as a calculated column was to bypass a performance issue (30 seconds to calculate as a measure) - i.e.just to calculate it once at report initiation. I've managed to tune the formula such that it runs in 1 second (instead of 30) - ... thanks to Marco for his Optimising DAX Course in London!! so I'm now able to run it as a measure for now.

I suspect I know why I am getting the circular dependency when I try to set it as a calculated column, but I can't for the life of me figure out how to get by it. I think it might be because of the variables I'm using inside the SUMX to materialise certain data early  to reduce the number of rows being crunched by my iterators. I think I've isolated the problem to the line that is marked -----YYYY in the code below. I couldn't figure out how to use Alberto's ALLSELECTED work around, and the row ID's didn't work.

It runs in DAX Studio just fine as an ADDCOLUMNS(....) but when I try to add it as a calculated column on my table I get

    "A circular dependency was detected: Payments[Column]"

Here's the code.  I gave it >4 hours of my life trying to make it work!

Column = CALCULATE(
    SUMX (
        Payments,  /*-----YYYYY Think the problem is here */
        VAR MyCurrency =
            CALCULATE ( VALUES ( Currency[isocurrencycode] ) )
        VAR MyDate = Payments[Payment Date]
        VAR FilteredXrateTable =
            TOPN (
                1,
                FILTER (
                    CALCULATETABLE ( Xrates, Xrates[From] = MyCurrency ),
                    Xrates[Valid from] <= MyDate
                ),
                Xrates[Valid from], DESC
            )
        RETURN
            Payments[Gross Local]
                * CALCULATETABLE ( VALUES ( Xrates[Exch. Rate] ), FilteredXrateTable )
    )