What percent tint to put on what windows?

Been reading up on tint and the ceramic stuff seems to be near miracle working on heat blocking.
At any rate, I was going to call someone and make an appointment but when I walked in and looked back at the truck it appears to me the back window is already tinted?
Are back windows pre-tinted at factory?
If so, for those of you who have done matching tint, how'd you know what % to put on what windows?

Thanks for any clarity/feedback/suggestions. Cheers,

 

Most SUV and trucks rear glass has factory tint. Basically just a privacy glass. No heat rejection. Factory is about 18-20% I believe. I originally had 20% on front door only and it matches the rear pretty well looking from outside. From inside can see difference of film vs colored glass.

This is 20% Xpel XR ceramic on front only. Stock rear.


Eventually I took off the Xpel and went llumar pinnacle ceramic 15% all the way around. But back glass turn to 5% (limo tint) with 15% film over factory tint rear. And added 5% brow. So rear door looks darker at 5% now than front door 15%

 

My tint guy said it was tinted also. But I believe it’s just lined with a shade of film. The sun still gets into my son’s eyes while driving and it annoys me. And you can still easily see through when looking from the back. Planning to get a darker ceramic tint for it eventually

 

I to did the Xpel ceramic 30% two fronts and 20% rear windshield.

 

I got 76% Ceramic Heat Rejection on my front doors and windshield. 20% to match the rears on the front doors and 50% for the windshield. I'm not sure if I had higher expectations on ceramic with heat rejection but since we aren't in summer anymore it's hard to tell how well it does. I was too cheap to spend $100 more for 96% heat rejection.

I think I might remove the 20% and have them do 13% and then have them tint over the 20% rears to make them match closer to 13% just to get it a little darker.