Personally, I made the mistake of going 5% once for window tint years ago and I swore I'd never do it again. WAY too dark IMO. You can't see jack outwards especially at night. It looked silly too. Tacky IMO.
I think that factory tint is something like 18% or round thereabouts. I typically tell my installer "darkest legal limit" and I've always been happy with the match and performance.
Same with the windshield, lighter than you think is almost always better because if you're not happy with it, removal is a PITA (read: expensive if you don't do it yourself). I think most of the windshield-specific tints are like 50-75% and they do a magnificent job cutting down heat/UV even if you can't really even see it.
Just some opinionated thoughts from someone who has tinted dozens and dozens of cars over the decades.