Heatmap using calculated column ??

[JonCampbell]

Morning, 

I'm trying to create a heatmap based off a calculated field. From a simple table:

Column A - UK postcode areas

Column B - Count of customers in one month

Column C - Count of customers in another month

I'm then using the 'calculations' -> '% of previous column' function to generate a new column showing the % change between column B and C. Finally, I set all columns but the % change to be 'hidden in the visualisation'.  

Using a table visualisation, I'm left with a simple table - 2 columns - location data in the first, % change in the second. 

However when I switch the visualisation to a map, it fails to load correctly (and in fact still shows in the map legend Columns B and C as the data the map is presumably trying to work with).

Any suggestions? I'm guessing I could write a custom measure which would create the % change field... but feels like it should be easier than this?

Thanks, Jon