Shape layer resizing from non-centered anchor point

I'm sorry the title is confusing. I'm not sure how to describe this more succintly, but an example will tell you what I'm after:

I have a shape layer, a rectangle with rounded corners. I want it anchored from the top edge and I want to maintain the same radius of the rounded corners no matter what size changes I make to the rectangle. The size property seems to ignore both the layer anchor point and the (smaller) shape anchor point. Changes to the size property are anchored in the center of the shape. All I can get to work is an expression. It works great, but there must be an easier way that I'm missing! Can anyone tell me an easier way?

Here's my expression solution:

On the Rectangle Path 1 > Position property

x = content("Rectangle 1").content("Rectangle Path 1").position[0];
y = content("Rectangle 1").content("Rectangle Path 1").position[1];
y += .5*content("Rectangle 1").content("Rectangle Path 1").size[1];
[x,y]

Essentially I have the shape positioning itself according to the size property, freezing the top edge in place no matter what height changes I make to Rectangle 1.

Any tips appreciated. Thanks!