Eye chart is just a handy ‘translator’ of how far you see clearly.
The lux for that, another variable that depends on your regular visual environment. Yes vision will vary quite a lot depending on lighting. If you change lighting, try to be consistent, keep a note in the log. Beyond that, it’s what you prefer and what environment you are looking to replicate / simulate when you do tests.
Again 300 lux is good indoor lighting. 🙂