try these sites, they might help. but you need to know stuff about your engine and preignition. what kind of fuel you use. you accually need to know a lot more then that. is this your first turbo? are you doing this yourself. there are books out too that explain these thing in lamen terms if you are not mechanically inclined. just dont want to ssee you blow the car up. http://www.rbracing-rsr.com/compression.htm www.turbofast.com.au/weclome.html
they have calculators and such on those sites i have more if you need them. it all depends on how your motor was built. i researched this subject for a long time for my car and decided there are cheaper ways to get horsepower, like displacment (for an m20 anyways). good luck and dont blow up your car.