Compute a rating of a mtd average compared to a rating scale
 

I have a monthly number of complaints. I need to compute a MTD average of
these complaints and rate them as excellent, good, fair. The problem is that
the rating scale changes based on the month

Month Complaints Excellent Good Fair
Jan 2008 20 <=20 21 - 30 30
Feb 2008 30 <=20 21 - 30 30
Mar 2008 40 <=10 11 - 25 26
Apr 2008 <=10 11 - 25 26
May 2008 <=10 11 - 25 26

I need to compute a MTD Rating of complaints only for those months that have
complaints recorded -in this case Jan-Mar 2008 (data for Apr onwards is not
yet available). I require this formula to be in one cell (hopefully).

Please help!