Fit Widget to constrained camera

Hello UE community !

So here’s my situation : I have a camera with a constrained aspect ratio (16/9), and a widget that should fit to this constrained camera view.
To give a bit of context, I’m working on a sniper simulation, and I’m trying to improve the reticle by keeping the reticle-to-capture ratio constant (so for example, if I look at a target in the scope and the target is 10 graduations tall at a certain distance to target, I want it to stay 10 gradutations tall no matter my screen resolution or my window size).

I tried many things to have the reticle widget follow the camera changes, working with overlays, size boxes to constrain the widget’s aspect ratio, manually setting the reticle size, but I couldn’t find any way to make it work, either my camera zooms out of my widget grows when I manipulate my window.
My first idea was to somehow get the camera view size and force the widget size to be this one, but didn’t work as well.

Any idea how I could do it ?