You are probably looking for a weighted average, but you need to do a lot of work to determine how important each component of your scoring system really is. Your current metrics do not appear to contain enough context or sufficiently-defined management goals to be useful without additional work.
The problem with any set of metrics is that you have to carefully define your context and provide meaningful goals. When you have imprecise metrics, you run the risk of measuring the wrong things or incentivizing the wrong behavior.
As an example, you want to measure "number of customers assisted." What if the value is 1?
- Is that because the person was unhelpful?
- Is it because you had very few customers that needed help?
- Are you trying to prioritize customer assistance over new features?
- Are you measuring this dimension against the rest of the team (i.e. who handled the most of the 27 tickets this period) or against some management target (e.g. Joe should handle 1/5 of all customer calls)?
There are many other contextual and analytical issues beyond these, but the preceding at least gives a good flavor for the sorts of questions you and the management team should be asking yourselves. There isn't a "one size fits all" formula because metrics require constant tuning and interpretation to be useful.