I am asking for assistance or guidance to point me in the right direction for this problem I am trying to find a solution.

I need to come up with a percentage to show a Severity of Defects for monthly software releases at my company. Each month our customers inform us of bugs/issues with the software and we create a "Report Card" to rank these defects.

There are 4 severity levels and each one has a different weight
SEV 1 has a weight of 1.00
SEV 2 has a weight of 0.75
SEV 3 has a weight of 0.50
SEV 4 has a weight of 0.25

My first attempt at it was a simple weighted average in excel. I used a dot product calculating the # of each SEV by its weight. The issue with this method is that it accounts for the outliers and this could skew the results if there is a high # of a particular SEV.

I think I need to steer towards some sort of Chi-Square Distribution. I also think I need some bounding built in to take care of cases with more defects than opportunities for defects. In other words, if you are counting only number of tickets entered vs. number of changes made and 10 customers enter a ticket for the same issue, does that count as 10 separate defects?

Any help would be greatly appreciated.