First, I apologize in a way for coming across like an expert. I got burned with bro science when I first got back into growing after a 20+ year hiatus. I planted 6 seeds in coco and pulled them all around day 15 and threw them in the trash. So I went back to what I know, which is soil, and I've been reading "What a Plant Knows," "Teaming with Microbes" (both of which I recommend), watching the extremely fascinating
Future Cannabis Project videos on YouTube (which can be upwards of 3 hours long), interacting with longtime growers on various forums, and watching
Dr. Bruce Bugbee's YouTube videos on growing cannabis. Just trying to share what I've learned. Definitely seek out different sources to cross-check what you are hearing, including what you are hearing from me haha. For example, here is an
auto grow guide from Mephisto Genetics, which is a highly respected breeder in the auto world. But there are some topics that are fact and not my opinion, such as that autos flower by age and not by light, and that autos are more sensitive to nutes than photos, generally speaking (some strains are more hungry, others less so).
You can see what I'm currently growing here:
BoCoGrow's Roc Bud Inc auto grow update
Sorry, you mentioned "plain potting soil" in your response to my post, which suggests you might be switching to a medium that is devoid of nutrients. Doing that is your call and I can't help there. Seedlings have enough food to cover the first two weeks of life, but from then on, they need to be fed. If you are going with a neutral potting soil, then you will need to figure out the right feeding regimen, which may be through trial and error. I use Ocean Forest, which tends to run a bit hot and thus burns seedlings. To correct for that, I fill my pots with Ocean Forest, and then I scoop out around 3-4 cups of soil in the top center and fill that with seed starter mix. I plant my seeds in that and that gives the seedlings a bit of time to get their legs before they hit the amended soil.
RE Build a Flower, I would suggest amending the soil when the plants begin transitioning from veg into flower. Roughly 1-2" across the soil. Build a Flower will get hydrophobic as well if you go too long without watering, so you can cover the top dress with something like barley straw mulch to keep it moist. Then, later in flower, you would add another 1" or so on top of the barley straw and cover that with barley straw again. A lot of folks recommend adding blackstrap molasses and other items, but again, given the 20+ year hiatus in growing, I'm trying to ease back into it and will experiment and augment my grows as I learn more.
This is all just scratching the surface by the way. There are topics such as fermented fruit extracts, fermented cannabis root ball extracts, use of humic acid and soy-based amino acids, etc. that people are using nowadays that still blow my mind when I think about it. Please do feel free to reach out with questions, though I may redirect you to someone else I know who is far more experienced than I am.
Edit: P.S. Build a Soil also has videos on YouTube in which they talk about growing, so since you liked how they went about creating their Build a Flower, you may want to check out their videos (and blog) as well.
. You sound like an expert in a good way, no apology warranted. 
Bugus Monkey's new and improved puzzle basement and various ramblings