You need to have valid reference value. Havent looked into your project yet, (just woke up and i am in work now).
As for mobile games, touch interface does not play well with umg input. UMG is made for one touch and mouse pointer, and touch interface is multitiouch.
For it all to work nice you need to code tracking of all touch locations, detecting gestures (if you need them) there are some gestures in unreal, but they are either weird or i do not undestand them yet.
For eg with standard umg button, if you press it then drag touch event outside that button will get stuck. Sometimes such behavior is wanted but mostly not.