I have always mixed FV 20/50 with FV Ultraboost at a ratio of 50/50. I also add a small spoon of Liberte’ full fat vanilla yogurt. This is what I began using when I had my first dwarf baby. He needed the extra calories from the Ultraboost. I know other rehabbers that add Ultraboost at a 25% to 75% Fox Valley formula ratio and their babies do fine.