There is no global place to set it, it needs to be set in each process where you want it to be in affect. So if you’re talking about a Phoenix app (which you are I see from above), then you’ll need to set it in each request (as a plug would make sense). And then in each other process like liveview processes.
The flipside of setting the context being really cheap: it’s fine to do it frequently, close to the code that depends on specific settings.
Setting it close to the calculation also avoids gotchas where code runs in another process unexpectedly; for instance, Cachex.fetch runs the supplied fallback function in an internal process, NOT the caller’s process.