If you have the fox farm id say you can wait at least a few weeks. And with the top dressing, I'd recommend a couple tablespoons spread on top, possibly as the plant get further along you may want to change to 1/3 + cup of dressing and then eventually change to a bloom like top dress the 2-5-3. Id start the 2/5/3 just before your flip to 12/12. And just watch for any deficiences to increase the top dress.
I love the sustainable organic grows, good luck, happy growing. :v:
NarwhalUp 
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Hey there I love the grow style you're aiming towards. Ive gotten so sick of everyone's habit of throwing out their soil because it's damaged by bottled nutrients, myself included. I'm moving into the reusable way, and after my grow store guys mentioned that fox farm was in with Monsanto i was a bit bummed. Since mixing my own top dress makes me a bit nervous with the measurements, I'm starting with this Mr. B's Green Trees, they're pretty similar to what you're trying to do i think. Good luck with the bag seed i hope it comes out female. :v:
@Experimentgreen, Thanks for the comment and like. I will most likely be changing my compost brand. I loath Monsanto and anything they have their greedy fingers in. For the bag seed if it turns out to be Male I'm still going to grow it out cause I want to try my hand at breeding.