Small roots blower for smallblock

How big of a motor are you running? I want to type the numbers in the calculator to see what boost numbers it comes up with. Depending on the eff% for my setup it says anywhere from 12-24psi. Stock 273, 144cu/in blower, 3" upper, 6" lower. If you put the numbers in for a 340 and it comes with a more reasonable number.