If you take two layers of ceramic tint in 30 (meaning blocks 70% of light transmission) what is a reasonable expectation, 

a. that the result is ~ 15
      simply additive
b. that the result is ~ 9
      meaning that the first layer blocked 70 and allowed 30 to pass (as in photons), and the second layer 
      blocked 70% of the 30 that passed the first layer, so only 9 (photons) passed, hence ~ 9% film

Huper Optik is the tint under consideration. 30 is their lowest, and they are, arguably, the best, unarguably German