Cut Down Rune

The Preaseason builder says Cut Down is 4% effective on enemies with 150 more hp than you, but only goes up to 10% at 2000 more hp than you. According to my calculations, assuming I pick a squishy support like and build minimal hp (Eye of the Watchers and Redemption), and i'm up against a hyper tanky with Resolve runes (+130 hp) and loads of HP boosting items (Face of the Mountain, Ruby Sightstone, and Righteous Glory), Cut Down will be 10% effective approximately... never. with aforementioned items at lvl 18: 2261 hp with aforementioned items and Resolve at lvl 18: 3536 hp Even if we throw in a Randuin's and Dead Man's Plate that still only brings her HP up to 4,231, just shy of the 4,261 threshhold for Cut Down to take full effect. She would have to build a Warmog's in order to cross that threshold. Even generously assuming Leona builds nothing but HP boosting items (the ideal circumstances under which Cut Down should be good), Cut Down doesn't become fully effective until lvl 18 and full build. That's way too late in the game to make a difference. As long as "10% effective at 2000 more hp than you" still stands I can't see this rune ever being effective for supports. Even if you're a squishy going up against a hyper tank. Who is this rune designed for? Vayne Tops?