This is Amazing but until now there is Not still an practicable way to Change the Rectangle Radius After creation like is well Explained in this Adobe Documentation+Comments.
So the Easiest way to make your Rectangle with preferred Rounded Corners settings is:
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Select the Rectangle Tool
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Do Not Drag but only Click on your Document Area
A window will Pop-Up leaving you to enter directly the Rectangle Settings…
So first try to find the exact Rectangle Size and after Try to Guess the Best
Corners Settings… :)
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