Customize KPI calculation in Thingworx 8.5

Hi guys,

I am trying to figure out how to customize the KPI calculations on a shape level and not equipment level.  Is this possible? 

For instance in my system quality is always 100%.  I tried changing the QualityRatio_Calculate service on the PTC.SCA.SCO.QualityRatioThingShape thing shape but that is not possible.  Where can I set this then because it has to be true for all assets.

Any assistance is welcome.