You guys can throw out all the fancy engineering formulas you'd like, but in the real world there is no way you can figure it out mathematically unless you can take into consideration the efficiency of the compressor at the boost levels you are seeing (i.e; how much heat it will be generating to do what you want it to) , friction losses from the intake plumbing, efficiency of the intercooler, IATs before the blower, AFR etc...

When this question gets posted on most boosted forums, I have just about always seen guys post that they are seeing somewhere in the neighborhood of 18-20rwhp per PSI up until about 15 psi and then you start seeing some diminishing gains.