Try this. Make a rectangle. Then pick the "white arrow" or Direct Selection Tool or keyboard shortcut A from the toolbar. Click anywhere on the canvas but the rectangle to deselect it [this always help to see the anchor points rather than path].
Hover your cursor over 1 corner, it will show a white box and the word anchor. Click on the white box to select that anchor point. In the top area below all the menu, you should have more tool options. One of the options is Corners. Next to Corner, you can input the value of your corner radius. This is where you can modify the corner radius exactly and not by the bezier handle. You can also shift click to choose 2 or 3 or all 4 corners and modify them together.
Another neat trick is if you click on the word "Corners:" you can choose different type of corner style such as inverted or chamfer.