I use only HPS bulbs my experience has been using the light spectrum in a sodium bulb (More Red for flower and More Blue for veg ) is much better then the HID not to knock the HID its just what I've learned over the yrs.
I can only speak from what i know and that being said the size of your buds are one part genetics and one part overall heath of the plant, proper lighting and defoliation IMO and experience allows the flowers of the plant to grow faster/bigger then with plants that have dying leaves shading your bud sites LOL. I know that might open up a big can of argument but it is not meant to start argument.
I would say that the flush is the point in which you give your buds their last little push and most genetics play a huge roll.
I'm not sure that helps or not but its all i got in response
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