Service Contract Pricing - Performance Based Pricing?

Hello,

We are in the process of implementing Oracle R12 and I'm struggling to find information on how I should enter a specific type of service contract.  We are servicing hardware in the field for our customers.  Some of our service contracts have been negotiated as "performance based pricing" and the value we can invoice depends on the uptime in the field.

Typical situation for the service contract might look like this:

Service Product:  SRA999  Item Maintanence and Support  UOM:  YR  QTY: 1
       Sub-Line:  Item instanance in ISB for the customer
Billing Cycle:  Annual, Arrears
Negotiated Pricing: 

Uptime = 100%             $100
Uptime = 98 to 99%      $80
Uptime = 95 to 97%      $65