The question and desiderata
I'm looking for a specific number here, even an equation. At what # of reported coronavirus cases (or estimated actual cases) within a population should the members of that population start self-quarantine measures (e.g. isolating themselves, avoiding contact with elderly, wearing masks)? Please state your reasoning, cite your sources, and/or show your math.
I know that being excessive in viral response is a good thing, and we should err on the side of doing too much, but I'm trying to be optimal in accounting for the behavioral costs of such measures.
Utah as a case study
Utah has 1 reported case. (source) It has a population of 3.161 million. The transmission rate of coronavirus in the US is around X (I don't actually know if there's a good source for this). Given these numbers for Utah, what is the "trigger point"- the ratio of (COVID-19 cases / population) at which we should self-quarantine? For example, we should begin self-quarantining when the number of CV cases reaches 1% of the population or 316,100 cases.
Or more fine-grained, what are the specific series of trigger points at which specific measures (mask-wearing, self-isolation, eliminating public gatherings) should begin?