I've done something similar with Probability Score and Impact Score to give me a Risk Score. I created custom fields for Probability and Impact with 5 drop down selections - Rare, Unlikely, Possible, Likely and Almost Certain. The drop downs are given a value based on where they are in the list - for example Rare gets assigned a 1. The Risk Score calculated field than becomes PROD(Probability Score,Impact Score). Hope this helps.