This is a lot easier to do than to describe. Beats the heck out of winding the hex nut with my thumb and finger in my experience!
By using a piece of tape to hold the hex nut in place, you may more easily tension the magazine spring when winding it, using the entire back plate as a tension winding device to optimally tension the spring.
A piece of tape is also handy to hold the feed lip insert in place while you tension the mag spring, while winding the spring with whatever method you use.
If your magazine already feeds fine, and you are satisfied with the spring tension, next time you dis-assemble the magazine for cleaning, tape down the hex nut in place first. That way the nut will already be in position to re-tension the spring using the backplate to wind/re-tension it to the original state that existed before you dis-assembled it.
Start by positioning the rotor all the way in its upright position, so it rests against the metal feed lip insert. Then position the backplate top end about 90° to the left, and insert the hex nut so it's spring tip hole will line up with the rotor’s spring tail with the backplate in that position. Then tape the nut in place. Now push the taped nut on to the rotor spring tail to engage it in position to wind up the spring.
Now rotate the entire backplate clockwise 1-1/4 -1-1/3 turns, using your thumb and middle finger, all the while keeping your index finger on the taped hex nut, pushing lightly against the taped hex nut to prevent it pushing out against the tape.
For a little more tension, moving the hex nut clockwise one or two hex spaces, so the backplate top is faced about 180° opposite of top, will give you about 1-1/2 - 1-5/8 turns.
When the backplate is wound around all the way 1-1/4 to 1-5/8 turns, until its top is in the upright position, seat the backplate/hex nut assembly onto the magazine body. This may require a bit of “wiggling” to get it to seat correctly, keeping your finger on the taped hex nut. Then insert and tighten the screw, and back it off about one 1/8 of a turn for rotation clearance. You may also choose to leave the screw loose in place if desired, while tensioning the spring.
I sure hope this makes sense from this old duffer!
SAFE and HAPPY Shooting my friends! Joe