Not enough buttermilk.......?!

Not enough buttermilk.......?

I'm about to make a cake and it says that i need 1 and 1 3rd cups of buttermilk, btu the thing is I only have about a cup of buttermlik. I was wondering what I can do about it, does anyone know if you can add regular milk or maybe water to make it 1 and 1 3rd? or will that completly ruin it?

Here are several substitutions I've found:

Buttermilk Equivalents and Measures
? 1 cup buttermilk = 1 cup yogurt
? 1 cup buttermilk = 1 cup milk PLUS 1 Tbsp vinegar or lemon juice (let stand for 10 minutes before using in recipe)
? 1 cup buttermilk = 1 cup milk PLUS 1-3/4 teaspoons cream of tartar
? 1 cup buttermilk = 1 cup water PLUS 4 Tbsp powdered buttermilk
(reconstitute before using or add dry to dry ingredients and wet to wet ingredients before mixing
? 1 cup buttermilk = 1/4 cup milk PLUS 3/4 cup yogurt

Personally I've only used the Milk & Vinegar substitution, but it worked fine. (I used it in biscuits.)

Good luck!