Naeddyr has a tutorial in the tutorial forum for something highly similar to this, as I recall. Let me see if I can find it.

Ah, here it is:
http://www.cartographersguild.com/showthread.php?t=5025

This is the projection we're all talking about, by the way: http://en.wikipedia.org/wiki/Equirectangular_projection

I don't know too much about projections, though. Naeddyr seems to be the resident projection-nut, from what I've seen, so I'd ask him.

-asp