Viewing size and distance
This is a measurement method, not a product specification.
A viewing-distance figure is a geometric preference, not a prescription. Comfort depends on the picture, your vision and the room. This site will not convert a resolution into a mandatory seat, and it will not claim a health benefit.
For the screen itself, compare TV technology for your room .
The relationship used here is horizontal field of view. For a chosen angle, distance follows from screen width: the width is divided by twice the tangent of half the angle. The width comes from the screen diagonal and the aspect ratio, not from the cabinet. A 16:9 screen is the usual case. If the manufacturer’s page gives a different ratio, that ratio should be the one in the calculation, and it should be labelled as such.
Two labelled presets are available in the tool. Thirty degrees is described as a wider room view. Forty degrees is described as more immersive. You can also set a continuous angle inside the supported range. The presets are selectable preferences. They are not a standard that every room should hit.
Going from distance back to size produces a continuous diagonal. Nearby size classes are shown with the angle they actually achieve at that distance. The result is not silently rounded up to a larger class, and a size that is not listed is not described as available.
Use the distance tool together with the cabinet measurement, not instead of it. A screen that occupies the angle you want can still be too wide for the alcove or too deep for the stand.