If the design you’re working on requires a rectangle with rounded corners, don’t use Adobe Illustrator’s rounded rectangle tool. Instead, use the regular (square) rectangle tool, then go to Effect>Stylize>Round Corners in the menubar. From there, you can set the exact roundness amount you want, as well as enjoy the option of adjusting it later via the Appearance panel. One more advantage of doing it this way is that you can stretch the rectangle box later without altering the roundness of the corners.
