CSS - Mobile button appears in different areas, different devices

I have a button in my hybrid mobile app which when viewed on an iphone 5 is in the area i want it (middle centre).

When i view it on a iphone 6 plus it moves around, now i know this is because of my css which is very specific but i was wondering if i could get some guidance how i can position a button which remains the same between devices (same location).

This is my css, all help appreciated :)

#divheader {

    position: relative;
}

#btn {
     position:absolute;
    left: 10px;
    top: 198px;
    text-align: center;
    color: #171e28;
  background-color: #f4af03;
  border-color: #ee9f05
}