Park tool says to go to small ring, large cog and get 1mm of space between the derailer and chain. Shimano says to do the same, without cable tension set, and make the limit between 0-.5mm. Problem with Shimano is that when you put tension on, it closes the space up. What is a good place to set that low limit so A) the chain does not fall off on shifts, and B) you don't get rub in the low-low position.