another thing just to add, that will give you a really close answer, but if you can figure out your exact atmospheric pressure at the time of the dyno you will get much closer resualts.
14.5 or 14.7 is thought of as the average, but it can vary from that depending on altitude and weather conditions. in a science lab we've calculated the atmospheric pressure to be as high as 16psi with little humidity at near sea level. higher up, it can be as low or lower than 13.
engine design also makes a difference, so far i'm talking N/A. actual atmospheric pressure on the back of the intake valve will be closer to 11psi than 14psi N/A because of heat, and various other things dealing with the engines design.
an example, i'll use boost vs displacement. boost can be thought of as a multiplication of displacement almost. a boosted 3.0l engine should have similar output as a N/A 6.0l if the 3.0 is on 14psi. most times the boosted engine will out power the larger N/A engine because 14psi is higher than the actual in cylinder pressure of the N/A engine. in this case it would be 3.0l with about 26 true psi and 6.0l with around 11-12 true psi
let me know if that doesn't make any sense at all.