How to calculate the reasonable allowed number of mistakes

There are four sample formulas demonstrated here. Would one of them fit the circumstances?

Guide to Computing Margins of Error for Percentages and Means

define "reasonable". What's the cost to the company in terms of time and $$$ when she makes a mistake? Are each of the variables equally important? Is this a homework assignment or a real life application that you've been asked to summarize?

You can take each record (invoice) and score it as a binomial variable based on all 7 entries (mistake/no mistake) and then calculate a proportion and binomial confidence interval around that proportion, but you still need to define "reasonable".

Or, you could weight each of the variables based on some factor of cost (time/$$$) when it's wrong and look at the weighted average of mistakes made. What's the standard you're holding her against? If perfection is ostensibly required and you discover that she's making mistakes 20-30% of the time then you don't need statistics to tell you that you've got a problem. OTOH, if this is your benchmark for future employees then you want to be careful in establishing the definition of "reasonable".

It doesn't make sense to me to compute what is reasonable for a typical person with 5-10 years of experience based solely on data for a single person. I can see using her data to determine if she's improving or getting worse, but how do you use her data to compute the norm for the general population?