Just had tint added to front doors using 50% (max allowed by state law here) and had the rear glass tested. He used the same device the state patrol uses. Reading on rears was 28% on my white '08 4Runner Limited. The difference between the two levels is hardly negligible.
Maybe a little off topic, but I wasn't looking for vanity glass; instead needed something to block out UV rays as I've had a number of bouts with skin cancer the last two years.