I've done it so many times now got it down to a science: wait until the 5th set of nodes then top/FIM. You'll get the first set of baby leaves from seed; those don't even count because they die off. Then you'll get your first set of true leaves; those don't count either. Then you get your first set of true nodes that's one...then the very next pair of vertical branches is 2....keep waiting until you can see 4 sets of nodes. When the 5th set emerges like a set of praying hands that's a good time to top which will predictably split the main cola into 2...do it again a couple weeks later to get 4 colas. If you allow the bottom branches to become the same height as the main colas they will also become new colas; it's all about making the canopy even to keep the buds a uniform size. If you FIM you'll get 3-5 tops in one shot which is my personal preference especially if you plan to scrog. Keep on topping all those tops to achieve desired bushiness.