I don't understand the dimension you want. Looks like the top of the spinner sits on the Y axis, at the center of your duct. The X axis is radial to the duct, so your dimension is the duct radius?
Probably going to need to create a reference sketch or plane to use as a mate reference. You would end up editing your sketch, not the mate, to change the position of the item.
What is the purpose of the dimension?