Fixed Resolution for mobile VR

Hi everyone,
Here is a link to a project we are working on:

It’s a gazed based experience and the issue we are having is when the VR version is viewed using a cardboard on an iphone 6/7 or Galaxy s7 the focus is decent - the gaze targets line up. However, if we use a Galaxy 8, which has a wider and higher res screen the gaze target is way off and causes a double image.

Is there a way to fix the size of the VR output?
I hope this makes sense! Any insight would be greatly appreciated!