After many attempts, it appears, as advised, the best method is to simply do the calculation in the UNX. It would seem logical to have this available within Design Studio. Luckily, I can get access to the UNX, but others may not.
I created a Percentage object in the UNX:
cast( 100*count(field1) as Decimal (18,8))/ sum(count(field1)) over ()
I bring this in with the response dimension and get the percentage as a whole number.
Thanks for the replies!