I FIGURED IT OUT! I also feel so incredibly dumb. I stumbled upon it while trying to show you what the screen resolution is. It turns out that when I tried subtracting half the screen resolution before, I was using the wrong number. Here is the functional version:
And here is the broken version I had been using:
Thank you so much for helping me though! Even if I did end up rubber-ducking you in the end 
Edit: Adding here because I marked this one as the solution, I ended up moving the code from the HUD to the widget itself (everything remained identical), and grabbing the viewport size every time instead of setting it in “Begin Play” because otherwise it would just be 0,0.