1. Identify the focal length of the scope. This is often marked on the body of the scope or it should be given in the user manual. For example, the Nipon 26-78x78 scope's focal length is 780mm.
2. Find the focal length of the eyepiece. For example, for the PL16, PL26 and PL32 eyepieces as listed at nipon-scope.com, their focal lengths are 16mm, 26mm and 32mm, respectively.
3. Divide the focal length of the scope by the focal length of the eyepiece to get the magnification of the scope. For example, if you wish to use a 16mm eyepiece to replace the 26-78x zoom eyepiece of the above Nipon scope, the magnification of the scope will become: 780/16=49x.
It is desirable to have a range of eyepieces with different focal lengths to allow viewing over a range of magnification levels.
Please keep in mind that, as a fundamental law of optics, at higher magnification powers an image will always become dimmer and less sharp. With every doubling of magnification you lose about half the image brightness and 3/4 of the image sharpness.
Therefore, it is best to begin viewing with the lowest power eyepiece (with longest focal length, eg. 32mm of the above example) or with the zoom eyepiece being adjusted to its lowest power level. This will provide the widest true field of view which will make target finding and centring much easier. After you have located an object, you can switch to a higher power eyepiece (with smaller focal length) to see more detail (if atmospheric conditions permit). If the image you see is not crisp and steady, reduce the magnification by switching to a longer focal length eyepiece, or for a zoom eyepiece, zoom out. In general, a small but well-resolved image will show more detail and provide a more enjoyable view than a dim and fuzzy, over-magnified image.
Please refer to "How to calculate the field of view" for more related information.