15-35-15 for the NPK reading is way off for vegitative growth. You want high N for veggin. Low P, and a medium K...Once flowering starts you want an increase in P & K with the same or slightly less N.

Your soil mixture seems like your on the right track. I prefer perlite and Spagnum Peat mixed with organic potting soil. With the perlite and spagnum in a 1:1 ratio mixed with the potting soil in a 1:3 mixture, the 3 being more potting soil..