I have this 20-20-20 soluble fertilizer from Miracle-Gro. On the label, it said how much to use for outdoor plants, sth like 1 tbl/gal and how much to apply per square feet. Also labeled how much to use for indoor plants, which is much diluted potion. Question is: what about container staying outdoor? I will be specific, how much would you use per 10 inch basket growing wave petunia? 3 plants per basket. At what dilution and what frequency? I know petunia is heavy feeder but I have this constant fear of over-fertilizing. Thank you all experts!