Pick what users feel
Share of requests that succeed, and share of checkouts that succeed.
Pick a realistic target
99.5% or 99.9% over 30 days. Each extra nine costs far more than the one before.
Know your budget
99.9% over 30 days allows about 43 minutes of failure. Use the uptime calculator.
Use it
Budget left: ship. Budget gone: slow down and fix reliability first.
The Google SRE book notes that users on an imperfect phone network often can't tell 99.99% from 99.999%.