Scale to Ratio (HUD) how do i use it?

hi i have a problem with my hud. I wanna use a helmet overlay for my hud and it works fine with 1920x1080, but not with 1280x720 etc.

I follow the instructions from this tutorial: [UE4 Wiki - Tutorial ][1]
I create this function like this one in this tutorial. But how do i use it now? (Attachment)