In attached assembly I need to add a vertical dimension between two sub assemblies as shown in red in the following image.

But it seems I'm not able to do it.

I've tried simply selecting two outer circles, shift click, ctrl click, Smart dimension, vertical dimension. But none of them allows me to add the dimension I need.

Any kind of advice is much appreciated.

Edit: I've already done a work around explained bellow by Kevin Chandler

I'm interested in seeing if somebody can do it without a work around
thank you