Astronomy
My 10 inch Mirror with a 1/48 P-V Wavefront Error @ 550 nm
The human side of this interface is the "entrance pupil of a dark adapted eye," which is given as 7 mm. However, it is known this can vary by as much as 1 mm. To accomodate this varience, the maximum exit pupil of the telescope should be set equal to the entrance pupil as given above + 1 mm. Thus d$$_p should be set at 8 mm.
Eyepieces for optical equipment vary from 2.5 mm to 55 mm in focal length.  Now comes the best part: While the telescope can have only one FR their are two formulas for it. Thus:

FR = focal length of Objective in inches / Diameter of Objective in inches = f$$_e mm / d$$_p mm
To accommodate both the maximum focal length of the eyepiece, and the maximum entrance pupil of the human eye at the same time, an optimum FR of 55 mm / 8 mm = 6.875 is required.