#10. "RE: Nikon D3200 focus - Viewfinder vs Live View" In response to In response to 9
Having the viewfinder dioptre wrongly adjusted can't make the actual photo unsharp.
But if you want to adjust it, the best way is to remove the lens, turn the camera on and point it at a plain, pale-coloured wall. This makes the AF markings much easier to see. Then as you say, just turn the adjuster until the markings appear sharpest to your eye.