Calculate apparent distance with magnification in Google Maps Api

Question

I have a map, and I need to calculate the distance above and below the map, which has the zoom value and the height of the map.

Do you know some pretty solution?

Update:

I am writing an example, solving my own question.

Code: https://github.com/CrisalixLabs/crisalixlabs.github.com/tree/master/map_utilities/visible_area

Working example: http://crisalixlabs.github.com/map_utilities/visible_area/