How dark is too dark?

I’m getting my windows tinted this Monday and still haven’t decided on 20 or 15%. The 3’s cockpit already doesn’t have that might light coming through, so I’m worried if I go for 15 it’ll be too dark. I want it to be dark enough on the outside where it’s difficult to see in but not out.

Any advice from 3 hatch owners who have both 15 and 20? Or experience with both?

Side note I’m deciding to not get ceramic because it’s not really worth it for me.