Affectionate bearded one in Essex says - this needs some maths! You have to take the effect of all components - bore diameter, stroke, piston type (to dish or not to dish), combustion chamber volume (ideally measured, not read from a book) and compressed gasket thickness in order to calculate your compression ratio. You only need to do this if you are building a serious engine from scratch.
By the way, the P90 head is the "turbo" head and has all the valves moved up by 2mm to give a bigger combustion chamber (reduce compression). The UK cars with P90 heads have "flat-top" pistons to restore compression to normal for a 280ZX. In very simplistic terms, if you want a 2.8 with high compression, use N42 head with flat-top late pistons (great for normally apsirated) and if you want a 2.8 with low compression for turbo, use P90 head with dished pistons. Note that increasing bore size also increases compression ratio. More maths - I can highly recommend "Auto Math Handbook" by John Lawlor (HP Books 1020) if you want to get into this properly......