A phone screen is not a good substitute for a gray/white card when setting custom white balance in-camera.

A gray or white card works because it reflects the same ambient light that is illuminating your subject. The camera compares that reflected light to what should be neutral and calculates the correction needed. A phone screen emits its own light, so it does not represent the color of the scene lighting. In practice, the phone’s light can also mix with or overpower the ambient light, making the result inaccurate.

So for Nikon preset/custom white balance, a phone screen generally won’t work properly.

For post-processing, it’s a bit different: if you include something known to be neutral in the shot, you can use it as a reference later. But for that purpose, the important thing is that your lighting and white balance remain consistent.

If you need an improvised in-camera reference, a simple white sheet of paper is usually a better option than a phone screen, even if it isn’t perfect.