You can salvage as much of the levain as you would like. You can keep to a 1:1:1 ratio – starter:flour:water or you can do something less. Personally I typically will go with 1:2:2 or 1:3:3 – starter:flour:water. Just make sure you have enough for making the size batch you want to mix tomorrow.
If I left my leavin out for too long do I need to throw it out or can I feed it again to use for tomorrow? Â If I feed it again how much water and flour what I use?