To calculate the power of magnification of a microscope simply multiply the magnification of the ocular lens and the magnification of the objective lens. For a typical compound microscope with a 10X eyepiece and objective lens of 4X, 10X, 40X and 100X magnification, your microscope will have 40X, 100X, 400X and 1000X magnification depending which objective lens you use.
The same principle apply to stereo microscopes, a 10X eyepiece combined with a 4X objective lens will produce 40X magnification. Some stereo microscopes equipped with continues zoom objective lens with magnification from 0.75X - 7.5X. The total magnification will be 7.5X to 75X when combined with 10X eyepiece. The total magnification will be 18.75X to 187.5X when combined with a 25X eyepiece.
To maintain useful magnification with satisfactory clarity and resolution, you must avoid empty magnification or making the specimen appear bigger but not clearer. In general, total magnification should not exceed 750X-1000X the N.A. of the objective. For example, with a 40X, N.A. 0.65 objective, the total magnification should be between 480X and 650X.

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