sorry… twirl down the z position properties for the null layer and the layer you want to link. then select the z pos property of the layer that will be linked and choose add expression from the animation menu.
this should bring up another line of options/icons under the z position. in that line there is a pick whip, drag that pick whip to the z pos property of the null. now there should be some text in the field to the right of the pick whip (and other) icons for z pos. that property is now linked to the propery of you null.
if you add your multiplier to the end of that text it will offset the z pos of that layer in respect to the z pos of your null.