I ended up changing my time field to total seconds and then changed my LOAD statement to this:
interval(Turnout/86400,'hh:mm:ss') as Turnout,
interval(Response/86400,'hh:mm:ss') as Response,
interval(Travel/86400,'hh:mm:ss') as Travel,
interval(Service/86400,'hh:mm:ss') as Service
As a result, my final expression was:
I formatted my KPI field as a percentage and had exactly what I needed. Your help was greatly appreciated!