I've figured out the coordinate adjustment for the Caves map. Simply subtract 30 from both the X and Z axes as displayed on the Caves map to find the in-game coordinates. For example, if you place your mouse over a spot on the Caves map and get the coordinates x = -100, Z = 70, to find that spot in the real world you would dig down at x = -130, z = 40.
 I take that back, it doesn't appear to always work that way. I think the amount you have to subtract has something to do with the depth of the cave, but I'm not quite sure how it correlates.