To raise MgR/I closer to 21 you either need to add mass around the top of the grip or remove mass from close to the tip. In your case, I would try adding 12 grams at the top of the handle. This should get you pretty close. For a quick try that is easily reversible, you can simply tape two quarters (about 6 grams each) at the top of the handle. Tape them tightly, one on each side. If you like it, you can just unwrap your grip an inch or two and wrap some lead tape around to replace the quarters.