ok so you used miracle grow. if its healthy, and you do it RIGHT, technically you could take out 50-60% of the soil and replace with perlite.
i did it, it works, and CAN be done in the first week of flowering.
also, you say you want 2-3 or whatever oz's. well, to make that much bud, you would eat alot more nutrients than whats in any MG soil.
it may say on the bag "supplies nutrients for 3 months" but, it doesn't say "supplies nutrients for dank fucking chronic"