Here's how I do it.
There are 80 cups in 5 gallons. There are 96 1/2 teaspoons in a cup. This means adding a 1/2 tsp of (sweetener, concentrate, spice, whatever) to 1 cup of cider is about the same (-ish) as adding 1 cup to 5 gallons.
So take a cup of cider and add concentrate in 1/2 tsp increments. When you get what you like, add 1 cup of concentrate to the 5gallon for each 1/2 tsp you added to the sample.
OK, the math is a little fuzzy here. It works if you just want to ballpark sweetness. If you want more accuracy, add 1/2 tsp increments to 6.5oz of samples of cider.