In what proportion do you add the kefir or yogurt to your recipes?

 To the recipe I gave earlier in the dough rise problem forum, I'm reducing 1 cup of whole wheat flour (I found out I made the recipe wrong).But how much yogurt do you think I should add to it? Do I substitute the milk with yogurt/kefir ? Is the yogurt/kefir good for all whole grain recipes?

Thanks for the help.