How to get sum of countdistinct in dashboard formula

I'm trying to write a formula to get the distinct customer count divided by the total distinct customer count on a dashboard grid. For one value I'm using sum([Total Due]) to get the per row total and dividing that by w(Total(Sum([Total Due])), partitionBy(), orderBy()) to get the % of the full total.

This same logic won't work for the distinct customer count with:

CountDistinct([CardCode]) divided by:

w(CountDistinct([CardCode]), partitionBy(), orderBy())

This compiles but returns an error when displayed. Is there any way to get the distinct total customer account so I can do this calculation?