the iS144 is perfect in 10-15knts with 8,6 - 9,6 and a 48 to 50 fin (Nobody in competition uses a 52!! except more than 100kg). In your weight and 8,4 I would go down to a 46cm fin and you will be surprised how the combo character will change to :-))) !!!
iS121 perfect sails are 7,7 up to 8,6 and fins app 43cm-46cm. This combo is the next size down! We use this combo in competitions 15-25knts