I just finished an Auto AK grow and my plants got very large, about a meter total with LSTing, and one went a full 93 days from seed. I never went above 500ppm until about halfway through flower and then I only made it to about 800. If you're in a three month time-release of miracle grow I wouldn't add anything until you see a deficiency. If you see your plant exhibiting anything besides lush, green growth and your environment isn't screwed up then you can bet it's a nutrient problem. You'll probably be okay as being a sativa-dominant autoflower it needs next to no nutrients, so the miracle grow might just take you to the end if it doesn't burn them.
