Point prevalence to 95% confidence interval?

My point prevalence is 20% (2/10) so we captured 10 mice total and 2 were positive. However, that's a very small sample size so I can say we have a point prevalence of 20% but that doesn't really tell me what the prevalence is in the population. I used software and I got from about 4-54%, how do I do that by hand?!? I tried a few equations but since the sample size is so small they're not really coming close. According to openepi I need to use a Fisher's exact 95% confidence interval or a Mid-P Exact (the software recommends the later). (Assume the rodent population to be large.)