I believe that is indeed a good guide for when it makes sense to have a SRE team supporting a service or product (with the caveat that the number probably isn't $1MM).
There are also good patterns for ensuring you actually have adequate SRE coverage for what the business needs. 2 x 6ppl teams geo-graphically dispersed doing 7x12 shifts works pretty well (not cheap). You can do it with less but you run into more challenges when individuals leave / get burnt out / etc.
There are also good patterns for ensuring you actually have adequate SRE coverage for what the business needs. 2 x 6ppl teams geo-graphically dispersed doing 7x12 shifts works pretty well (not cheap). You can do it with less but you run into more challenges when individuals leave / get burnt out / etc.