Self rising flour is usually a little softer than AP, but not quite as soft as straight cake. This makes for a very tender biscuit, or a slightly airy cake. 1-1/2 tsp baking soda is far too much if you're using double acting -- which you should.
To make a "rough" cup of the equivalent to Bisquick:
1/2 cups AP
1/2 cups cake flour
1 tsp Double acting baking soda
2 pinches table salt
(Optional) 1 pinch Baking soda
Double acting baking powder responds to liquid and heat. Baking soda responds to liquid acid. Baking soda responds to liquid acid. The double acting baking powder contains a dry acid which converts to liquid acid when it's wet and then potentiates the baking soda in the baking powder (to produce leavening carbon-dioxide), and a pyrophosphate which is potentiated by heat (leavening carbon-dioxide again).
If you want to follow a "self-rising" recipe it's very important to use a double acting powder like Calumet or Clabber Girl, but not a single actor like Rumsford. The tiny bit of additional baking soda helps to kill any baking powder taste by fully converting the dry acid -- self risers are often balanced this way.
Hope this helps,
BDL