Determine percentage of target based on two dates

I'm trying to track a metric regarding meeting artifact delivery committments and need assistance in calculating the percentage between two dates - Target Date and Delivery Date.

Metrics
Marginal : metric less than .90
Review : metric between .90 - .93
Effective : metric between .94 - .97
Good : metric between .98 and 1.00
Outstanding : 1.00 plus 10% of commitments delivered ahead of schedule


Any thoughts on how this might be accomplished?