Is there some type of conversation for boost to horsepower? How do I tell how much HP will be gained from a certain boost

no. there are too many variables like compression ratio, cams, headwork, ignition advance, fuel... just like any NA motor.

To give you a ROUGH idea, take your naturally aspirated horsepower and devide it by 14.7, then multiply that number by 14.7 + X amount of boost. So say you had a 200 horsepower engine, and you were adding an 8 lb turbo.

200/14.7 = 13.6054
13.6054*(14.7+8) = 308.84 HP

That's just a rough guesstimate, that doesn't take into account that if it was a centrifugal blower it won't hit peak boost when you were normally hitting peak horsepower, parasitic loss from driving a blower, etc, etc, etc.