Ok, so maybe this information is already public knowledge...or maybe it falls under the category of common sense. All I know is I tried it thinking it was a good idea...
######the horror...I think I'll have nightmares of that match
So be smart and take a real rune cause PtT is really awful on Lux ;_; (it sounded like such a good idea though D:)
Jbels3 votesWhy would a keystone that revolves around autoing three times consecutively the same target sound good on Lux?
Final Spark Lux1 votesWell the 12% bonus dmg from all sources sounded nice. As a support player, making everyone else do more damage sounds very lucrative, not to mention my own damage as well. I thought if I started my combo with an auto and spaced them out like I normally do, it would be fairly effective for my damage as well as my teams. I don't think it would be bad in a team fight since I'm usually around the ADC peeling, and after I use my spells I'm just stuck right clicking, but I didn't get a chance to test that theory since our game turned into a 4v5. That being said, after trying it even in that horrendous game, I don't think it would make up for the amount of damage I personally lose from using proper runes.
Also for whatever reason I had it equipped in training while I was testing another rune, and after hitting the bot with a few rounds of autos, it said I had done 300+ damage to him, so I was like :0!? but I should really know better, since it's a bot and they always take a ton of damage, and they aren't going to react to a Lux right clicking them like real players will lol.
Infernape1 votesAt that point you should've gone AD + crit lmao
Final Spark Lux1 votesI did think about getting something along the lines of hurricane or rapid fire cannon at one point, but then me and MF got blown up a couple times from jhin / karma combo and kat roams, at which point our MF RQ and it was 4v5. I went supporty because our WW and wu were fed. I'm convinced that we could have won that game, but everyone else wanted to FF. Oh well. Never doing that again lol xD
Stars Shaper1 votesThat thing is good for characters who can do continous damage and thus also taking advantage from the true damage rune from the Domination tree.
While this is surely true for most of the ADCs, I think that others like Rumble or Azir might make a use of it (but this is just me brainstorming 2 possible AP characters so don't jump on me if it's not true).
ChiTenshi1 votesF*ck, I knew I was doing something wrong.
Best go back to Predator Lux.
Final Spark Lux1 votesProbably. It's really good for roaming :D
Critmaster Garen1 voteswell. it takes 3 auto attacks to proc, as opposed to the other keystones that need any combination of 3 spells and attacks.
coup de grace is still amazing for her, which makes precision a decent secondary tree. dont know if she needs presence of mind. its an option. otherwise triumph is amazing too.
what i really want to try is klepto lux. if you catch someone with an e or a snare, you get some extra gold/items when you go in for a passive proc with an auto attack.
dark harvest into a lichbane build for massive auto attacks between/after spells should be good too.
Final Spark Lux1 votesI can say from experience klepto is really fun on her if you don't mind forfeiting some damage for it. I'd use it more, but every game is just a DPS race at the moment, so taking it as a support puts me at a disadvantage in terms of raw power. Speaking of damage, I've tried coup de grace on on her, and I feel like the amount of damage it does over the course of a game isn't worth going into the precision for, depending on what trees you normally use. I've been using a lot of domination / sorc and vise versa, but that's because I support, so I really like having access to zombie ward / ingenious hunter to clear wards. I've compared coup de grace against scorch, and early game, scorch is just always gonna be better. Chances are if they're low enough to take 10% additional damage from all of Lux's spells and you have all your spells up, that enemy is dead anyways.
My thinking behind it is 10% of my ult is 30 damage not counting AP, and the flat damage from scorch is 30 minimum, but it applies every 20 seconds and I don't need them to be at 40% hp, so I feel like it's more useful, at least in the early stages of the game. I like that dark harvest idea. I think I'll have to give that a try tomorrow, and hope it doesn't turn out like that PtT game. I should download the reply just to remind me never to do that again. shakes head it was so bad...lol
Dark Nephthys1 votesThe only support that can really use it well would be 
since she can just activate it with Auto - Q - Auto and has the CC to keep target in place long enough for you to get it on them. That and adding in your own passive makes it really good on her. But no, I don't think it'll be very good on Lux or most supports (Maybe Braum can though).
Final Spark Lux1 votesYeah I've thought about it on Leona. She's actually my 2nd most played; I just haven't touched her since preseason started because I'm having too much fun with Lux :)
Hochelaga1 votesWhat about Janna? She can poke ennemies decently and apply the extra 12% dmg for her allies
Dark Nephthys1 votesShe really wouldn't be a good pick for it, even worse off then 
at using it since Janna has less range, sure she can poke a decent amount when she shields herself and walks up to auto attack but so could 
with her E and 
can even give herself attack speed to make procing press the attack easier.
With 
she gets a lot of great uses out of Aery since she shields her carry a lot. Janna would lose way to much if she went PtA.