i think the 3-4 level on the meter is an OK time to water if its at the very bottom. Towards the top you want it to be pretty dry because that is where the water will disappear first.
I have not heard good things about miracle grow and cannabis and would avoid them if possible... Superthrive is nice to help build strong plentiful roots, and real beneficial after transplanting. I would recommend only using any additives once the plants are healthy.