I have 35% on my back windows and 25% or 30% on the canopy rear window (bought it off Craigslist, forgot what they guy said). The back windows alone are fine, but the canopy is WAY too dark for my liking. Add my auto dimming rear mirror and it is terrible. My truck is fairly long so it may be different but you cannot back my truck up with just the rear view or looking back. I can only use the side mirrors.
What is the goal with the tint? I did a mirror tint on my old canopy so there was tons of light coming in and it was easy to see with but very hard to see into. It wasn't like having mirrors as windows either as I didn't like that look, it was just a shinier window. Below are a couple pics of my windows on both trucks.
Front windows no tint, rear cab windows and canopy side windows 35% tint, canopy rear window 25% or 30% tint:
Front windows no tint, rear windows mirror tint:
Working on getting other pics, phone doesn't want to upload right now though. I'll edit when I get them to upload! I'll also take some at night to get you an idea.
EDIT: Here's the other pics. The one looking back looks easy to see out of but it's not lol. Hopefully it'll look more like it actually does at night, right now it's an average cloudy day out BTW.
EDIT 2: This is with my spot lights on, without them you can't see anything. You can see the spot lights I have on the bumper in the first pic I posted in this reply.