- at most 20min per 3 hour

- and 99.9% uptime on a yearly basis

as in your yearly budged of outage is ~8.76h but that budged shouldn't happen all at once and if there is an outage it at most delays works by 20min at a time, and not directly again after you had a downtime

but I did fumble the 90.21% part, which is ~35.73 days i.e. over 857 hours....