Regarding "theoretical best", I think that is "in the absence of mitigations". I think you can build a service with a higher SLA than one of its dependencies, but only if you recognize that impedance mismatch and build in defenses.

As a contrived example, if you've got a microservice that provides data FOO about a request that isn't actually end-user critical, you can mitigate your dependency on it by allowing your top-level request to succeed even if the FOO data is missing. Or maybe you can paper over blips of unavailability with cached data.

But, yes, know what you depend on and how reliable they are, then see if you need to take more action than that if your target is higher than the computed target.