The apical growing tip is in fact the upper most point and flowers form at nodes not the tip which would be a vegetative collection of undiferentiated cells or whatever it is biology describes it.
But i digress, calyxs form eitherside at all points of the nodes and thus with calyx comes leaf.
I say the first calyxs to form are also the first calyxs to mature, generally the base of each node and then radiating upwards in each cluster.
Top bottom of plant is irrelevant, i cut at same time, the lower popcorn can form a little latwr than the rest of the bud but anything with enough light is rockung to the same cycle.
All this is obvious if you look at really well grown plants, often when my preflowers mature the calyxs which formed next just above on the base of each bud cluster brown a few days later and this is obvious to see if you crane your neck up and stare at the base of each bud.
The tip of each bud cluster which forms a bud generally still white or last to mature on a lot of bus but at 99% mature this is the 1% that isnt.
I hope you can see what im saying here, stress will alter your buds in many ways so carefull your identifying natural patterns not newbie patterns which i have had frustrate me with shit bud before.
Piss your plant of enough and calyxs wont swell and pistils brown orematurely or nort at all, thats just a crap shot