I've got lots of crab apple juice and also quince juice in the freezer and want to use one or other instead of apple pulp in recipes like Blackberry and Apple Jam, Plum and Apple Jam, and also to reduce the amount of raspberries, strawberries, blackcurrants or gooseberries in their jams.  My questions are - how much juice do I use per pound (or kilo) of fruit and how do I know how much sugar to use?  I'd love to learn from the experience of others rather than possibly waste valuable fruit by experimenting!