Building Calculated Measure to get percentage of a 'Sum' calculated Measure

[jhuisman]

I created a custom measure that pulls the Sum for a specific field using the following formula:

Sum("NumberFieldName", 'RowGroup')

This works perfect but now I need to take the value of another existing measure and divide it by that to get the percentage. So I created another Calculated Measure with the following formula: 

"ExistingMeasure" / Sum("NumberFieldName", 'RowGroup')

This does not return any data and I am not sure why. For the 'Summary Calculation' I have 'None' selected. Not sure if I need to use 'Custom' there or what I would put in there for the Summary Calculation if I did use 'Custom.' 

Does anyone have any suggestions on how I might get this to work?