Well, using Add instead of Multiply kinda works, however I get these really bad jaggies:
Also, the values are still a bit off. Using 0.75 didn’t really get me a 3/4 of the circle, it’s a little bit less than what it should be. I guess I can fix that by giving it an offset, but I think the radial mask you’re talking about is the way to go.
My mask also has sRGB unticked, so that shouldn’t be the problem. When I choose TC_Grayscale the gradient gets really smooth, but then I can’t use it in my material.