Krismuss, EFA is spot on, there is no accurate calculation. Only way to do it accurately would be to bench dyno your engine.
Unfortunately different dynos give different readings and the different dyno technicians decide on assuming different losses. The more % losses they assume the better your engine looks to be performing.
However in the real world the power at the wheels is all we are really interested in. Suggest you see how it goes and then when you decide you want more power and you upgrade it, go back to the same dyno and in that way you can compare previous results.
If you get someone who knows what they are doing and you get the engine re-mapped on the dyno you will find you get a much smoother power delivery. If you are running a busa the powercommander is a good first option to allow the engine to be mapped. Following that you can upgrade to an MBE system.
Rob.