How can I insert the value of some arbitrary dimension in a model into the callout for another dimension on the drawing?

Currently I would like to do this in order to call out the depth of a slot.  There's no slot depth variable so I'd like to put something like the following in the slot callout:

 X  

Where  inserts the value of the extrude cut feature depth from the model.  I imagine there is some way to do this with a global variable?

Other instances I can recall in which I would have liked to be able to do this include adding a far side countersink line to a hole callout where the countersink was added with a revolve cut instead of with hole wizard (somebody else made the part, not me).