Determining Sample Size for Quality Audit

I hope I am posting this to the right section, I really have no idea about statistics.

I work for a healthcare company working in residential aged care facilities (nursing homes). We want to audit the quality of our documentation. We've determined the items that we want to audit, and their weighting to provide an overall percentage score of the quality of the documentation for a single resident. What I am struggling to figure out is how many residents files we would need to audit to have an accurate representation of the quality within all resident files (represented as an average of all the percentages).

I've been looking on the internet, but none of the calculation guidance seems relevant to our situation. A lot seems to be for financial auditing, or auditing on things that have a yes/no, pass/fail or present/not present response, whereas our result could be any percentage. People that I've spoken to who work within hospitals doing quality audits are unable to give me information on why they audit the number of files they do, for all they know it's just an arbitrary number.

It's important for us to be able to explain to our clients the rationale behind our audit process so they are assured that our quality process is sufficient. I am aware that it may be possible that calculations will recommend a higher number of files be audited than is practical for us to review, but if we know those numbers we will at least be able to make an informed decision. And if we have a calculator or formula it would be possible to calculate the impact on confidence levels if we reduce our sample size (we wouldn't want to reduce the confidence level to the point that the whole audit is almost pointless).

Here's a little bit about our service and needs:
We work within facilities that have anywhere from 20 to 200 residents, so I will need a formula/calculator that I can enter data into, and a generic rule (like 20%) won't work because I believe the population size will have an impact on sample size.
Another thing that may have an impact is that we intend to complete these audits at 3 month intervals. Nothing that I've come across seems to take into account how frequently you audit, so perhaps this has no impact at all.

At the end of the day, I'd like to be able to use a calculator/formula to determine recommended sample sizes for a 95% confidence level, but equally be able to play around with the values so that we can make an informed decision.

I hope this has made sense and I've given you sufficient information to be able to help me. If you respond, please dumb down your terminology as I really don't understand statistics and I feel quite confused with all the things I've read.

Any help you could give me would be greatly appreciated - Thanks in advance!