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Disclaimer:

I am a random person trying to figure things out for myself using a spreadsheet. I take no responsibility for the accuracy of the numbers or the veracity of the equations. I am not an expert in anything related to this.
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How to use this spreadsheet
First, make a copy of the entire document (File -> Make a copy). Then proceed with the following:

"True" #cases given #deaths:
If there have been any COVID-19 deaths in your area, use this sheet to calculate likely upper and lower bounds on the number of COVID-19 cases in your area. If there have been no deaths, use ""True" #cases given #reported cases" instead (you will have to input numbers for column C yourself).

Risk from gatherings:
Input the population of your area in cells M2->O2 (highlighted in yellow), and the size of your gathering in cell K2. Then use the "true" number of cases that you got from the previous calculation, and find the closest value to that in column B. Reading across that row will give you the current best-guess situation in your area. (Note that cells where the chance that someone at your gathering has COVID-19 is between 1% and 100% are highlighted in red, and that rows for which the value in column B is larger than the population of your area will be nonsense.)
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