I Love cannabis! ...let me add a little more here please!
Looking at your grow you would be using it right now.
Not evident on the firsts grows, but in no time you'll be a pro at spotting stages.
You want to use when they are bulking up. This happens a few days like a week after end of stretch, and just before maturation begins.
Now what happen during this time: if you are not providing enough nutrients, you would start to see a few minor deficiency here and there, mostly on the top and bottom of plants. You are OK. Giving more than they need is a waste. It won't grow bigger buds. This is strain related and light related. Than your nutrients comes in to play to *support* the growth.
So if you have on hand, keep it.
If you see a minor trouble on some leaves, it should be a sign they need something more: usual suspect in flower are K, P, Cal or Mag....
It all comes down to observation of your plant to spot any issues.
In coco it is very easy to correct a deficiency.