I just bottled some cherry melomel, and I ended up using 100% cherry juice. (It was tart cherry juice from Trader Joes.) I had considered not going 100%juice and had tried to calculate out what 3-5 lbs of cherries would like like when converting to juice. I had several people convince me that the flavor would be better just going straight juice. I think I could have used some water, as it's expensive going 100% juice, but I have no idea where the break point is that more juice is overkill.
But in my apple wine/cyser I like to back sweeten with concentrate, so it comes out to more than "100%" juice now that I think about it! I do know I like doing 1 gallon batches for the most part so if I make one I don't like there's not too much. I only have a few recipes I'm ready to scale up to 3 or 5 gallons on.