When it comes to farming, i know the experiences differ.
For me, that's what i would conclude after my experiences farming at both Nuri and Sewers.

1) When luck comes, it wouldn't stop, and you should really keep farming.
- It is quite true for me anyway, I used to farm a lot at Sewers. And once I farmed a RR recurve and the luck just wouldnt stop. I farmed another RR leather recipe and 2 other pinks within 3 hours.
-Yesterday, I first farmed a lv60 Strongman Glyph Mace at Boggy Waters and I believed it was my lucky day and i kept potting up. I farmed another 1 glyph wing and 1 bracer (although they werent part of the sets). I potted up for 3 hours and got 6 pinks including 3 glyph. All my teamates were like astonished.
-In contrast, I have tried to farm for 6 hours non-stop with elixir but it yielded nothing, not even a lot of crap pinks.

so my theory is when you first farm your glyph item, no matter good or bad (part of the set, if that's how you define it), keep going for it, keep potting up if you like. If that's your lucky day, the luck just wouldnt stop.
I am not sure how luck is calculated, and many say its only a random number. My experience says that when the luck comes, it usually lasts for a certain period and a couple of premium pinks can be farmed.