Any idea what gasket thickness/deck height they use to get this compression?

[bull]

http://www.campbellenterprises.com/mopar_speed_pro_trw_forged_pistons.htm (see the 383/.040 over section)
PN: L2315F 40 -- Ring Groove Widths: 5/64 -- CID: 391 -- 7.85cc: 9.81 -- 88cc: 8.98 -- Comp. distance: 1.920 -- flat top -- press fit pin

I was checking out some Speed Pro pistons for sale at Campbell for my 383 and they list the compression ratios at 9.81 on their 78.5cc set and 8.98 on their 88cc set, but they don't give you the whole story. Is there an assumed deck height and head gasket thickness that they use to make these calculations? If so what is it? This is with stock, non-hp heads, 906 I believe.

I sent them an email already asking about this but I'm just wondering if this info is common knowledge?

Thanks