The FOV for a viewfinder on a rangefinder would typically be a little wider than the lens so that the photographer could see what is just out of frame.
For an SLR (digital & film), the FOV would be equivalent to the lens, but potentially with a slight crop ("coverage"). FOV is equivalent because you're looking through the lens, but the crop exists because the light gets bounced with mirrors and the mirror & ground glass housing might slightly cut off the edges.
For an EVF, it would be nearly the exact same concept as the SLR. The FOV is the same as the lens on the camera.
If instead you meant the ideal viewing angle of the screen, then that would depend entirely on the screen.
If you meant the distance from which the screen should be viewed, that would also depend on the size of the screen and the optics used in the housing of the EVF. Calculators for TV screen optimal viewing distances exist that you can use as a guide.
If you meant magnification, again here I don't think this is a very relevant consideration for a display, but you should be trying to achieve a 1x magnification.