What is the best Fraxel Dual 1927 setting for dark spot/melasma? (photos)

I am a Caucasian, (Fitzpatrick Skin Type III-IV). I had done 4 Fraxel treatment for melisma/dark spot in an anesthesia institute which charging base onf the number of pass. I don’t see noticeable difference. Each session, they do one pass of 1927 laser and the setting is Energy 15 mj, Treatment level 11, Passes 12. which of the following options does make my treatment more effective? Doing two passes in each session? Decrease setting passes# from 12 to 8or 6? 1550 laser instead of 1927?