Unless you have an extra slab for your ceiling, I wouldn't change its reference line. If you have just one slab between your stories and you fix your ceiling plans by changing the reference line, it will mess up your level dimensions on regular plans.
And I'm not sure if the Level Dimension Tool was really meant for dimensioning undersides (in RCPs for example).
I use Label tool with Associative Geometry Method. There, you can specify what you want to show - in your case "underside".