Oooh I love my Easiyo. I make one batch according to the instructions and when we have nearly finished that batch - with about two or three tablespoons left in the bottom - I fill it up with milk. Fill half way and shake, then fill to the top.
Only ever bought two packs of mix. I also tried doing the same with milk powder which worked okay too - just followed the milk powder instructions for making 1 litre of milk. So leave a small amount of the last batch, add in the milk powder and fill with water half way, shake and top up.
I also found adding a couple of tablespoons of milk powder to milk made a gorgeous thick yoghurt. Yum.
Good luck Mtilly!