Similar idea here on my Lelia 2.0 & R5M, I just use model paint to paint a couple of the notches. I paint one orange and the other white - on the .25 mags it’s an odd number of pellets so the two painted notches aren’t directly opposite each other. When I install the mag, I rotate it so that the orange notch is on top closest to the action. So when I’m shooting and the white notch get up to this same position, I’m about halfway through the mag. When the orange gets back up to that spot, I know I’m out.
Alternatively, on my R3M before, I would just fill in the exposed empty slots on the mag as I shot so the mag would always be mostly full.