Just stop the miracle grow,there no reason for a taper off type thing just get a better light and some flowering nutes around 10-40-10.
MJ will not flower well under that light and you will regret not geting an HPS...get a few friends together to buy it and share your first harvest with them and then buy it from them later...if this is possible.Believe me you'll kick yourself if you don't get a better light!