I got it to work (I think), as I came up with the same numbers as the calculator Frank posted. I put in the following:

8.5 : 1 static compression ratio
20 PSI boost
0 ft. altitude

So pretty much, ((20/14.7) + 1) x 8.5
((1.3605) + 1) x 8.5
2.3605 x 8.5 = 20.06
20.06 : 1

Again, I'm not sure if this is correct but it came up the same as the calculator.

'91 LWB, 31x10.5s, KYBs, A/T "I look like Robert De Niro, I drive a Mitsubishi Pajero" - Billy Bragg (not the original lyrics, but that's how he sang it when I saw him in Dublin in '04)