Ascorbic Acid (Vit C powder) with Baking Soda = Single Acting Baking Powder?
This answer suggests that you can substitute ascorbic acid for cream of tartar at a 0.75:1 ratio.  However, the answer does not cite how it arrived at that ratio.  It's not an unreasonable idea though, since the pH of ascorbic acid is much lower than that of tartaric acid -- that is, it's more acidic.  I lack the chemistry math to say if the 75% substitution is the right number, though, so you may need to just test it out.