Control premiums can vary in size depending on the amount of control the shareholder has.
There are a number of studies that have been done to determine the appropriate control premium for a private company (Mergerstat Control Premium Study is one) so it’s not as simple as your example. Generally, control premiums are viewed as the excess a buyer would pay over a minority interest in order to acquire control of the company.
For example, if the control premium for a given company was 25%, your minority interest discount (or discount for lack of control) would be 20% (1-(1/(1+0.25)). Here, a buyer would be willing to pay 25% to obtain control of the company. In order to calculate what the freely traded value of a minority interest, you would discount whatever the controlling interest value by 20% (the minority interest discount or discount for lack of control).