Proportions for improvised cran-apple juice
Searching for "substitute fresh cranberry for cranberry juice" (because that's the actual substitution you need to make) gives you this nifty recipe for cranberry juice. 

Proportion is 1:2 w/v ratio on fresh or frozen berries as juice equivalent (100g berries = 200mL juice). 

CHECK THE LABEL of your juice to verify the ingredients just to be safe - if you have pure cranberry juice, I'd stick with 1:1 w/v ratio and then add more if necessary.