Sorry about the double post but I just got back from the arcade and I'm pretty sure I know mostly how it works now.
First of all, the LAs are no longer set amounts of damage, and they're all calculated by what appears to be a fairly consistent formula. I don't know if LAs that do stuff like circuit break, Roa's life drain, or Hisui's confuse. Anyways, they're based fairly linearly on how much health you have when they go off. Maybe this is only Full Moon, I don't know for sure. Oh, and Full Moon can NOT get LA off held shield. You get it off shield just because there is no ex shield, but LA will only come from a fast shield (maybe it's a bit more lenient than the ex shield timing in crescent, but definitely not held shield). It also MAY only work when your guard meter is gray or better. I mashed shield a bit and once got what seemed like a fast timed shield, but no last arc came out, and my guard bar was red.
I got off LA with Sacchin only once, and it did 7700 with a bit more than 2/3 of my health remaining. A lot more than I expected because I didn't know how it really worked, but it seems about right now.
I did LA with Nero 3 times. Once I was almost at no health, it did a little over 3000 damage. Then once with about half my health, it did 7400 to Vakiha, and once with about 60% health, which did 6800 to someone else I don't really remember (I think it was Riesbyfe).
I did LA with White Len twice. First time I had about 2/3 health left, and they had almost none, so it was an instant kill. Other time both of us had almost all our health (8500-10000 range) while and White Len LA doesn't display a damage number, but it did about 9000, probably a bit more. Oh, and it's slightly changed to where the big crack at the end actually does about 1/2 the damage of the total LA damage. Now it's not just
I did it with Neco Arc once. I was at full health. 13000+ damage, instant PA-FEKUTO-. Yes you read that right, 13000+ damage. Time for Jiyuna to go embarass some noobs.
I did it with Vsion once, should have done about 6000-7000, I had about half health. Also the animation is changed. Now she raises her arm like she's doing 623, then the screen fades to black. Both players appear out of the black, and a giant gray claw/beast thing is summoned in the background (it should be the claw of Dust of Osiris, except made out of stone). Then the claw slams into them for 1 giant hit. It doesn't display a damage number since it's 1 hit, but it was a Last Arc Finish and they had like 6000-7000 hp.