create a datum plane offset 50mm from a block face and mirror it... still associative.
strictly speaking, you should try to do what you are doing without creating two primitives.
i would do what you want by creating an associative line at your desired point on the cyl. you will need a tangent datum to make an associative intersection line:
you can create a datum plane tangent to the cyl face by:
create a first datum pl (DP1) thru the cylinder center by clicking the cyl...