uff, sounds like the system is scaling around the dice scaling up. It does not seem to translate too well to using dice with less surfaces, unless you scale up the number of dice... let me think...
a d6 is bringing results of 1 to 6 with an expected roll of 3.5.
a d8 is 1 to 8, expected 4.5.
d10 1-10, 5.5
d12 1-12, 6.5 ~ 2d6 (2-12, large heap around the 7)
d20 1-20, 10.5 ~ 3d6 (3-18, large heap on 10.5 - but that's GURPS)