Changing dimension "points" on plumbing items?

[madcowscarnival]

I don't regularly dimension plumbing infrastructure, but have a client asking for dims to help him with plumbing layout on a slab.  Using point-point dims, the "standard shower" will grab a center of the whole block, but doesn't recognize the drain location (slightly offset from the drain).  The toilet picks up a center that's roughly the center of the bowl, rather than the plumbing rough-in location.

 

Is it possible to somehow manually set where you want the dimension line to "snap" to on a plumbing (or other) block?  I tried to "explode" (I'm more used to CAD terms) it, but that button doesn't appear when the block is highlighted.

 

I also tried to "place point" but for the life of me I can't make it appear at my crosshairs when I'm visually placing it at a (say) drain (it shifts slightly down and right, sometimes up and left, its like trying to pick up an orby).  Plus, I would like to have the dimension point "built-in" to the block when used.

 

Hopefully it's something simple.  Any advice would be appreciated, thanks all.