I'm having some troubles understanding the concept of effective pixel if there isn't some kind of maximum bound we can sum to (like Bootstrap/Semantic UI columns).
Let's say I always want to have my app do something when the window is at half the width of the display resolution, and let's I have two devices, one is 1920x1080 and the other is 2160x1440 (Surface Pro 3 resolution). Effective pixels tells me I can design something for one device and it should scale accordingly for the other device. Given this information, and any other kind of device resolution, what number of effective pixels do I use to always trigger the said half-screen resolution event?
Microsoft tells me the "breakpoint width" for desktops is 1024 effective pixels, but if I set the event to occur at 512 effective pixels, I do not get my goal behavior. Reference https://msdn.microsoft.com/en-us/library/windows/apps/dn958435.aspx
TL;DR : More specifically, I'm trying to make an adaptive trigger fire when the window is half the length of the screen resolution, but I don't know what amount of effective pixels to use, or if I can use percentages instead.
There is no way to do this; you don't know how big the screen is (in any unit of measure) so you can't know how big half the screen is.
Some caveats:
The question I'd have though is why do you want the trigger to be half the screen? That is a huge range of sizes from a few inches / few hundred effective pixels for a small tablet to over a foot / a few thousand pixels for a big screen.