you are aware that the 2220 fish has a 20" transom.
the pro models come standard with a 10" bobs. the rear deck around the motor is designed alot different than the fish model so you need at least a 10" setback.
your fish model is designed to not need any setback. so you can use just a 4" setback so you dont move the center of gravity back too far.
however if you are looking to move weight to the back for more speed, ect. a 10" or 12" setback would work
2220 PRO/ 225H.O. E-TEC