As the others have said, the only way to know for sure is to give it a try. My own suspicion is that you could add 10% without problems, and maybe even 20%. Like many others, I have added UHT cream to a recipe to up the fat %, and it has worked fine. I'm guessing the "good milk" provides enough structure to help hold the proteins/fat of the UHT milk ... but I'm just guessing ...