Another way of looking at this by "first level approximation" is:
15psi = 2 atmospheres
8psi ~= 1.5 atmospheres

18 CR x 2 atmospheres = 36 CR
21.2 CR x 1.5 atmoshperes = 31.8 CR

So, I believe the calculator is accurate! I would never have thought that any engine would be able to handle a 32 much less a 36 CR. This is why it is necessary to reduce the typical 6.5L CR from 21.2 to ~18 to run 15psi of boost.