I've generally taken quality out of HACCP plans where I've seen it used. I think it gets confusing and misses the point. IMO if you are going to use HACCP for quality, you're better off using a different HACCP hazard analysis so you don't get distracted when considering food safety.
If you do want to come up with a score, I'd suggest a matrix with likelihood on one side with up to 5 terms (3 would be fine though, e.g. rare, occasional, frequent - it's important to designate what these mean though, e.g. rare = unknown or less frequent than once per year, occasional 6 monthly - 1 year, frequent, < 6 monthly)
Then on the other side you could have impact / severity. Again between 3 and 5 terms (use the same number as you use for the frequency.)
So you could have (with appropriate descriptions):
Mild - isolated customer complaint or customer dissatisfaction
Moderate - widespread customer dissatisfaction, small uplift in complaints (chose a number appropriate to your business, e.g. increase by 5 in 1 month might be suitable for one company, increase of 2 would probably be suitable for mine.)
Severe - Significant numbers of customer complaints (again chose a number), product withdrawal etc etc.
Then you can give a score to each. E.g. if:
Rare = 1
Occasional = 2
Frequent = 3
Mild = 1
Moderate = 2
Severe = 3
You then multiply the two factors in your matrix. Then anything on your matrix over 4 you might decide is a significant issue which requires control. I.e. on this matrix this would exclude anything which was a rare occurance or mild but everything else would require some kind of control (you can chose your terms and what number you chose so don't feel restricted by my example. Sometimes 5 terms matrices can be a bit easier and allow more grey areas so that might suit you better.)
