That matches to what I was calculating early on, during the time of peak panic when there were daily press briefings by politicians and "health" bureaucrats proclaiming how many new cases were detected each day. It was pretty simple, I was just using Bayes Theorem to calculate the PPV (Positive Predictive Value). This is a probability that, given a positive test result, that the result was a true positive. Using the data supplied by various websites, I could get how many tests were conducted, how many were positive, etc. and plug that in to a spreadsheet to calculate an estimated PPV. This was an estimate, because the true prevalence was unknown. My calculations gave me a PPV of around 0.1 - 0.15 for the period of time where they were conducting untargeted mass testing. Around 90% of the positive test results were false positives.