Interesting build, very similar to SporadicZero's BOOM HEADSHOT Lux guide that I just looked at. Also very similar points to mention.

I am glad that people post there builds/guides here but wish we had a little more explanation as to why you made certain choices and how its working for you. So without any comments I will let you know how I feel about it and await your response.

1. I'll just sum all the runes up with this one, I don't agree with any of your choices. If you want to stack flat AP go ahead but go all out, flat mana runes have been proven to be sub par to mana regen unless you are going to be returning to base to purchase far before levels 3-4. I personally prefer AP per Level as you will have as much ap from them as you do flat ap after a few levels anyway.

2. Lux is very squishy without a solid escape mechanism, so I would not advise Mejai's Soulstealer for any of her builds unless you are pub stomping.

3. With this build you have no Cool Down Reduction "CDR" which is arguably the most important stat for Lux next to AP. With the exception of when you get Golems and your Mastery's Intelligence and Sorcery, and even then you didn't take Utility Mastery to maximize the benefits of blue buff. With only 9% CDR from Mastery's and the 25% from Golem buff you are still at only 34%, or 6% off cap. Maybe you are always maximizing your Mejai's Soulstealer but in that case you should get Guardian Angel to avoid every losing it. Sort of a Karthus type build if you will.

4. You invested six mastery points into mana regen Perseverance Meditation which screams "Get Archangel's Staff early game to me.

5. I would take one mastery point out of Quickness and place it in Haste losing 1% overall movement speed for 8% increase and 1.5 sec duration increase when you need it seems like a good point spent if you are taking Ghost as one of your summoner spells.

Overall i'm not trying to bash all the other Lux guides, it just seems that way of late because I am interested in fine tuning my Lux so that is what I have been reading. I think if you comment with a good argument on these topics it will greatly help people understand your build. If you agree with some of my other anyone Else's remarks you can always change it in your build and your play and no one will think any less of you. It is the wise man that learns from mistakes and the fool who is always right. Likewise if you provide me with good sound reasoning I may very well tweak my own Lux guide.

As with the BOOM HEADSHOT Lux guide, I make you the same challenge. Improve your guide and site sound reasoning for why you have made your choices and I will change your down vote to a pleasant thumbs up that everyone enjoys seeing. Its not about which guide is the best or which Lux build would defeat another. It's all about making your Lux work the way you play her most effectively for you and your team.