so im going to be using fox farm grow big for veg and big bloom for flower. i have experience using these. i plan on vegging for 4 weeks then flowering (aiming for 12" plant height after 4 weeks). however my question is, say i get to 4 weeks and the plants are at, say 9-10", and i start to flower them and start using big bloom, what should i do to also increase their height while still giving them nutes for flowering? should i use a small amount of grow big with the big bloom? thanks.

It is unwise to combine any nutrient especially one for flowering and one for vegetation. Dont worry about the height, it will continue to grow after you switch light cycles and may double its size or more