Well, it's measured in two different ways.
1, the amount of light transmittal and 2, the actual percentage of the tint.
The 1st way tells you how much light actually makes it through the window.
The second way tells you exactly what percent the tint is.
Now, all the glass in our vehicles (or in any vehicle made today) comes with factory stained glass. It's not tinted, it's stained and at around 73-78% light transmittal. That means that the factory stains it so that ~ 77% of the suns rays penetrate the glass.
So, any layer over the factory stained glass is gonna make it seem darker. Now, I previously had 20% on my windows but I got it darkened to 10%. 5% is typically limo tint.
When you go to a tint shop, they're gonna ask you how dark do you want it. I told the guy that 20% was whats on there now and I want to darken it up.
So, it's 10%. It's friggen dark.
Attached is a shot of my tint meter from about 3 years ago when I first traded in my 04 and got my 07. These are factory stained glass windows and the tint meter shows the percentage of the stained glass.