I dont remember what screen does, but it would seem natural to simply Add lights, right? Actually in my scene I dont even notice any difference. What does it Screen do and how should it be different from add?
This is covered in the Texturing core course here: https://cgcookie.com/lessons/blend-modes-and-color-math at around 3:55 she covers that group of options.
I remember people saying in tutorials that Screen is a hack method and it breaks the under the hood math, so you should avoid screen. Always use Add because it doesn't break the math. Now if you ask me, use whatever looks good, screen is fine in my opinion. Unless maybe if you're doing very hardcore compositing and you're part of a pipeline and your team has a method to it all, and they're like use Add or we'll fire you and we promise you'll never work in the VFX industry again and we'll make sure our children keep an eye on your children so they also never work in the VFX industry. Other than that, you're good.
