How much/when to add nutro?

Depends on how long the miracle grow soil you added can provide a proper diet to the plant. You may need to wait for some symptoms to properly learn how and when to fertilize going forward in future grows, assuming you use the same products... start small and slowly increase over time.. observe and react to deficiencies or toxicities as you go. take notes... adjust what you do next time for better resutls... this is all you can do. Even if someone uses your exact same soil mix choices and fertilizer, you may find youneed to do things slightly different. your climate, lights and ambient co2 matter too. it all has to jive together. FF instructes incredibly high doses that will require frequent flushing. So, start at 1/4 strength and go from there. On the 'first' grow i'd wait till i see some hint of deficiency before fertilizing, but take not of timing so that i start a bit sooner the next grow. my suggestion is shit or get off the pot.. either go soil or soilless. Also, add perlite or similar to your potting mix. it rarely has enough out of the bag except for things like 70/30 coco+perlite. In a heavier soil you want 1:1 perlite. so if half your ptting mix is miracle gro soil and half is coco coir you need about 40% for a more perlite for an ideal gas:water mix and good drainage properties. Simply a weighted average. soilless is easier to learn. easier to diagnos. provides plant-read nutes that results in faster growth etc, as long as you are systmatic about learning and keep track of behaviour. soil has advantages too, they just aren't as important to me. YMMV. Regardless FF sucks a dick. LOL. I used the trio for one grow and gave it away... best choice i ever made. instructions suck, plants looked like shit... too esoteric and imbalanced to work well out of the box.

How much do you have to consider based on EC values or manufacturer specifications, from when? If your soil has nutrients, then it's enough from week 3 to 4. But it also depends on the soil; you can generally give CalMag from the start, around day 10. I don't know the fertilizer you have, but I give mine an NPK fertilizer with 7-3-5 starting from week 2. Then a bit later, I switch to a stronger one. But you just have to check how much EC your fertilizer mix has by slowly adding fertilizer to the water and seeing how the value changes. You can look online for reference values and phases of the plant.