The way I adjust my rear brake is to first loosen all three nuts at the rear.
The large nut and the castellated nut on the axle and the nut slightly forward of the axle that holds the brake backing plate in position.
I believe all three of these nuts must be loose so that the brake backing plate can move to a centered position.
With these three nuts loosened, I tighten the small adjusting nut on the rear of the brake rod that runs from the brake lever to the brake cam lever at the rear of the bike until the rear wheel is locked up and will not rotate.
This tightening will expand both brake shoes against the inside of the brake drum, centralizing the shoes and the backing plate with the brake drum.
I then tighten the large nut on the axle, the smaller nut on the brake backing plate and then the castellated nut on the axle.
With all of these three nuts tightened, I then loosen the brake rod nut until the wheel turns freely without any brake shoe drag.
After doing this, some people have found it necessary to adjust the brake light switch so it will activate when the break lever is depressed.
That all said, there should be no drag on the rear wheel when the brake lever is not actuated. There is no single value for the number of turns the brake rod nut must be loosened to achieve this. It takes what it takes and no more.
With the rear brake adjusted this way I find it isn't the most powerful brake in the world but if I really push hard on the lever, I can lock up the rear wheel.