Associating a Probability Percentage to a Field (Lead Module)

Associating a Probability Percentage to a Field (Lead Module)

Hi All,
 
Total noob to Zoho. We have gone live with the system, and most of the setup is fine. I had a question from my Managing Director : "Can we associate a probability percentage to the rating field in the Lead module".
 
After some inspection, I just cant figure this out - but surely this should be possible. You have this functionality in the potentials module (Sales Stage). Herein you map a percentage to a sales stage
 
example :
 
Sales Stage = Deal Lost ; Probability = 0%
Sales Stage = Deal Closed ; Probability = 100%
 
This is just a simple mapping example - however for the lead module we have a rating that you assign to a lead. With this rating we want to associate a percentage.
 
Our Rating options is a pick list of :
 
Poor
Fair
Good
Very Good
Excellent
 
How can I change this so it has a percentage associated like :
 
Poor                  =      10%
Fair                   =       20%
Good                 =       50%      
Very Good         =       75%  
Excellent           =       100%