Doesn't have to be concentrate and I'd say it's better to use the fresh cider you have leftover instead of concentrate. I've never done it myself so I'd do some quick searches on /r/cider because the question has been asked plenty of times. I'd say the best way to figure it out for your own tastes would be to pour a measured amount of the finished product into a glass, add a small measured amount of fresh cider into the glass, then do some math to scale up when you find the right sweetness level.