you can only figure that out by testing it on your boat.
stick it on and go ..lets say 60 mph, remember at what RPM you did 60.
then take the rpm X the prop pitch divided thru the gear ratio ,divided thru 1056 = XY
now you take the differents between the number you got and 60 and figure out how much % that is from 60. THAT IS THE SLIP # !!!!!
example... boat runs 5000 rpm and does 60 with a 23 pitch prop.with a 1:50 gear.
5000X23 = 115000/1,50 = 76666.666/1056 = 72.601009 ( theoretical speed )
72.60-17% = 60.25 MPH
so the slip would be 17% !
Hope that helps !