"Luck goes up in increments of 600 I seem to find.
600/1200/1800/2400+"
Interesting! How did you come up with that?
From hunting Tmaps, Peerless and other mobs, I cannot say for a 100% accuracy as the RNG in this game plays soo much but I did see differences, when wearing different amounts of luck and I may not of done exact figures, maybe 100 over in some cases, as it would mean having to craft pieces at different luck levels and that would be very expensive but call them chock points or tier levels, there was difference when overcoming this levels.
The intensity of the items (Number of Mods), amounts of Greaters VS Legendaries is where you can look, if you want to check it out yourself, now it means hours or doing it, to get a small idea of patterns.
Now about when in Party and luck on the highest two damagers taken into account, well thats a weird one, as doing roof, you would think the sampire would do the most damage and take most damage unless a tamer is there with their GD to tank damage, well seems even tho if party not broken and other non top damagers having luck, their luck is take into account, so not only the 2 top damagers work. (Sampire +Tamer/WoD mage)
Now also what many also do is take the monster to redline and then break party so that most the luck of the group is transfered to the others. Still not being top damager your luck in your own items is also increased and the other too without being partied.
So does luck work as its intended? Maybe at times, seems luck also has another hidden glitch that works when it wants.