1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x.
2: An exit pupil size (diameter of light beam as it exits eyepiece) over 7.5mm might be too large for telescope designs with central obstructions (i.e. Newtonian telescopes).
3: Exit pupils less than 0.4mm are impractical because eye floaters and eye-lashes can interfere with the view. They might also indicate the magnification is too high.
Calculate the magnification of a telescope and eyepiece combination. Formula: Telescope Focal Length / Eyepiece Focal Length
Determine the eyepiece focal length required to achieve a particular magnification. Formula: Telescope Focal Length / Magnification
Calculate the theoretical maximum magnification possible with a particular telescope. Formula: 2.5 X Telescope Aperture (Maximum: 350x)