The rear windows do measure to about 20% with a tint meter but if you put 20% film on your front windows I bet it would be slightly darker than the backs but the front windows will look matched because of all the light being let through the windshield (assuming you haven't put any film on your windshield)
The reason that I bet it will actually be slightly darker (to a tiny meter) is the front windows measure to somewhere between 70% - 75% not 100. I put 50% film on my front windows and they now measure to 35% to stay within NC law.
So if you aren't worried about that tint being illegal go ahead and get 20% film on. Hope this helps guys.
'11 Laredo / Bright Silver Metallic / V6 / QT1 / 17" Wheels / Tow
3M 20% tint up front, 255/70/17 GY Wrangler AT Adventures, 5000k LED interior and reverse lights
Audio: Pioneer AVH-X1500, Alpine MRX-M55, JL 12W3V3