Can you get water from an outside faucet that isn't connected to the RO system? And is does it have a softener too, because softened water is bad for soil and plants.
Build a Bloom already has Ca and Mg too, so you shouldn't need extra. As for dosage, it's 1 tsp per gal, but I've never used it personally. I have pretty much everything else they have though, lol. Just email them and they'll get right back to you with the answer you want.
And personally I would rather topdress dry amendments, especially outdoors. And some need meal would help with bugs and feed the plants, but that's more for veg. Karanja meal is the one to use in flower since it doesn't have a lot of N.
I've used Build a Flower top dress and it's good shit.