Calibrate your VR Device specifically to you using this tool to prevent nausea and headache from extended use of your VR Device.

There are a number of cheap and readily accessible VR devices out there, at the end of the day, they are just “containers” to let your put your mobile phone in to act as the VR screen.

Even though you can get by using your VR device without calibration, what happen is your eye end up conforming to the misalignment from your uncalibrated device.
It will work for a while, but depending on the individual, nausea, headache and disorientation eventually creeps in, this is due to that part of your brain letting you know you are cross-eyeing and or focusing unnaturally for far too long, this can all be avoided had you calibrate your device before you use it for game or movie watching, this app does exactly that and no more.

Calibration takes 2 min 38 sec if you have two separately adjustable lens.
Calibration takes 1 min 46 sec if your lens cannot be adjusted separately.