Here's the expression
a=content("Rectangle 1").content("Rectangle Path 1").size[0]*-.5;
b=content("Rectangle 1").content("Rectangle Path 1").size[1]*-.5;
[a,b]
So this is something I use all the time and I thought i'd share a quick video on how to do it too.
Note this only works on rounded rectangles but you can make it a regular rectangle by making roundness 0.
also the size[0] is calling on horizontal axis and size [1] calls on verticle
