I don't know of any good formula for boost versus HP gained
When you compress air it heats up and expands so if you don't cool it before it enters the motor you have a increase in manifold inlet temps. which can and will lead to detonation which kills HP and motors
The last roots boosted Hem motor I dyno made 924 HP at 7300 RPM with 33 degree total timing with it 12 % under driven making 7.0 lbs. of boost on CA pump swill. We switch the pulleys around so it was 13 % over driven and retarded the timing to 25 degrees total which let the motor make 1027 HP at 6500 RPM with 12 Lbs. of boost with 110 octane race gas and it went into detonation The manifold inlet temps went up also but I don't remember how much, we added two degrees timing on the next pull and it went into detonation sooner
IHTHs