How many external probe locations, and what failure threshold, make uptime alerts for a small site trustworthy?

Open question

External uptime checks for a small site are cheap to set up and hard to tune. One probe location reports its own network problems as outages of the site; several locations with a confirmation rule such as "alert when two of three fail on consecutive runs" trade false alarms for delay, and the delay grows with the probe interval and the number of confirmations required. Each refinement adds surface: a content assertion (a phrase that must appear on the page) catches a blank 200 but fires on a copy change; separate IPv4 and IPv6 probes double the checks and the failure modes; a TLS check adds its own class of errors; a check that follows redirects hides a broken redirect, while one that does not fails on an intended one.

The SRE book's monitoring chapter distinguishes symptoms from causes and states that every page should be actionable and that a person can react with urgency only a few times a day before fatigue sets in, but for a site with one origin, a few pages and no on-call rotation the practical parameters are rarely stated: how many locations, which interval, which rule, and how often the result was wrong in each direction. Is there documented practice, with the alerts counted over a period and classified as probe-side or site-side, showing which combination kept the channel trustworthy, and how the operators told the two classes apart afterwards (probe service status, a second tool, the origin's own logs)?

What a useful answer contains

The site's shape (single origin or behind a CDN, which address families), the probe service or tool, the locations, interval and confirmation rule, the counts of alerts that turned out to be probe-side and site-side over a stated period, how each alert was classified and by what evidence, the outages that were missed or delayed and by how long, and whether the rule was changed as a result. Vendor defaults should be labelled as such rather than presented as findings.