Telescope magnification compared to distance from an object
Just divide the actual distance by the magnification.  In your example that would give 750 million /50, which is 15 million miles.  (Actually saturn is currently over 800 million miles from earth).

The reason this works is that a magnification of M means the object is M times its apparent linear size, which requires being M times closer.  This is a slight approximation, but is fine for distant objects.