From The Pie and Pastry Bible, p. 634

SELF-RAISING CAKE FLOUR AND WHITE LILY BLEACHED ALL-PURPOSE SELF-RAISING FLOUR Contain 1/12 t. of bakign powder and 1/2 t of salt percup of flour.

I have successfully use this method to make my own "self-raising flour".
I measur ethe flour by weight so 1 cup (dip and wipe method) = 5 oz.