The positioning tool looks to have a couple issues while being used in Z.
1.) It appears to use relative movements. After sinking an object by 5mm, clicking off the object, and than selecting the positioning again. Z will now show "0" for positioning instead of "-5mm"
2.) After dropping an object into the build plate by xmm, when clicking off the object it duplicates the movement. Drop attached file -5mm into build plate, and select off the object. It will drop another 5mm (or whatever value inputed)