The best is make the reference for the middle planes.
How do I make reference for the middle planes can you please give me an example
Do it with a width mate.
So you're trying to mate the rounded square on the right to the 4 plate group to the left?
If you want things "centered up" I'd use planes.
If you designed the plates on the left to have mid planes or sketched them up with a centerline that you can mate to that.
Also have the rounded off plate to be designed with things centered up about a mid plane or center plane.
Then just mate all those planes together. That way, if the plates change size, it will always be centered.
I hope that makes sense.