I haven't used boosters in years, I'm on the KISS method now with Jacks start to finish.
When I did use them I would start when buds were popcorn size...so that's like week 3-5.
Then switch to the finisher.
I found more headaches than rewards with boosters over the years, burnt plants, Mg lockout and crap like that.
Just go easy with them and don't love your plants to death.
I remember back in the dark times when we grew for profit a buddy of mine fried a whole room of plants with H&G Shooting Powder.