Shadow cube map: Comparing the depth value

Hello,

I have a scene rendered into the six sides of a depth cube map from an omni light position using a 90°FOV.

So far so good.

Now in the fragment shader I somehow need to compare

A) the distance between fragment position and light position (direction vector length) with

B) the distance/depth stored in the depth cube map

Problem is, the distance B in the cube map cannot be directly

compared with the "real world" distance A, right?

How can I get comparable distance values?

I know how to lookup B but not how to transform the returned

deoth value into something I can compare with A.

Help is really appreciated!