How can I adjust brightness of 3rd Party external monitor on iMac, Macbook or Mac Pro via keyboard?
I've found Lunar.app and it syncs the brightness from your PC to the external monitors. It has a lot of other features that I not yet have played with, but a uniform brightness is already great. The app looks nice, so thats a plus (and it's free).

Here is an article on lifehacker.com, because the information on the Lunar site is really sparse.