My 15.0 hand paint weighs about 950 and he's fairly fine boned so that might give you an estimate of what your filly should weigh, since she's almost the same height and probably has bigger bone.
I usually go by the 2% of the horse's body weight and then adjust by eyeballing the horse. If your filly should be around 900 lbs., then feed her for her "ideal" weight. My colt was about 14.3 when he was under a year and I pretty much gave him free feed of grass hay and then a growth supplement. I don't know what he weighed at the time but I would guess, around 900 lbs. I would think if she is underweight, you could give her 24/7 good quality grass hay (orchard, timothy or whatever is in your area), fairly low in nutrients but it will help fill her out over time, gradually and not overwhelm her digestive system. Pictures please!!