Making baking powder substitute with baking soda and powdered citric acid
Yes, I have found several sources that say that citric acid is about 4 times the strength of cream of tartar. So, mix 1 teaspoon of baking soda and 1/2 teaspoon of citric acid and use a 1/2 teaspoon of the mixture.

That should work. Let us know!

EDIT: Oops, I should have mentioned this before the OP accepted. Hopefully, he'll realize, or see this. That substitution will approximate single action baking powder, so don't dilly-dally before cooking! (Difference Between Double and Single Action Baking Powder) 

2nd EDIT: Just to be extra confident, I compared the reaction (according to the method of David Lebovitz) of 1/4 tsp of my recommended mixture with boiling water and 1/2 tsp of new Rumford Baking Powder with boiling water. The results seemed identical.

3rd EDIT: I actually found this question pretty intriguing. While I could find plenty of evidence that it should work (including my own little water experiment), I couldn't find anything definitive that said it does work.

Well, it just so happens that I had some cream in the fridge, and I have been meaning to try America's Test Kitchen's cream biscuits. With nothing in them but flour, sugar, baking powder, salt and cream; they should be perfect for comparing real baking powder with the substitution.

SO:



I made biscuits.






They taste as identical as they look. (pretty yummy too)

I can now say with authority, the substitution works. 1 tsp fresh Rumford Brand Baking Powder = 1/2 tsp of a mixture of 1 tsp baking soda and 1/2 tsp citric acid.