iPhones can already tell when you are looking at them.

So they could automatically communicate with your glasses to tell them to go into iPhone focus mode. This would use your prescription but adjusted for phone usage.

And maybe in the future, Macs would be able to do something similar so your glasses would use your prescription adjusted for computer usage.

So instead of trying to determine distance by looking at eye alignment, which is problematic in some people with eye alignment issues, you just automatically switch between focus modes based on device feedback.