To my knowledge there is no standard formula or right level of reduction. In a perfect world you should have zero complaints meaning 100% reduction!!! However there are a few "rules" you can apply as appropriate.
First of all you have to filter out the complaints to relevant and irrelevant to food safety and/or quality. For example if you get complaints that your product doesn't taste like papaya, but you don't do anything like that, then it's an irrelevant complaint and you should just file it away (if you want for pure OCD reasons you can just register how many of those you get).
For the relevant now you should do an evaluation of how serious they are and what is their impact. For the serious (that can affect public health, the product having a wrong use-by date etc) you need of course a 100% reduction. For non-serious you can set up let's say a 10% or more reduction of complaints every year, be advised though that quality is subjective, some may like it some may not and some might just want to complaint for sport...
The above mentioned are rough examples to give you an idea. Bottomline is there is no specific rule (unless of course you want to go all pro and use marketing tools and statistical analysis for every single detail, which move a bit away though from the food safety part) and you need to do an evaluation of the importance of the complaints your receive and deal accordingly.