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Generally the focal length is provided by the manufactuer.
I have a 1200 mm FL scope and a 400 mm FL scope.
A 20 mm eyepiece would give me 1200/20 - 60X
Same eyepiece in the 400 mm FL scope would give me 400/20 = 20X
It is a simple ratio. The aperture has nothing to do with the magnification. Aperture tells you the relative amount of light the scopes will gather. It is based on the area of the lens/mirror.
Area = Pi R squared. Since Pi is the same in each calculation we can ignore it.
12" scope R-6 6 squared = 36
6" scope R=3 3 squared in 9
36/9 = 4
The 12" scope gathers 4x the light of the 4" scope so you can see dimmer objects and you can apply more magnification when using the bigger scope up to the point that atmosphere limits magnification.
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