Valve job on os valves

Ok thanks all for the good info i have a better idea of what i need to do now. I have another question about how much to shave off the head. I am keeping the stock bottom end and using a .040 felpro and want to have a target compresion of 8.5:1 but dont know how much to take off. I know there are calculators but shaving the head will affect teh cc and i dont know how to calculate the new cc.