I'm on my second 2L batch of this. The first batch, scaled the ginger down directly to 1/2 Cup. That was way too much. Tasted good, but was HOT. The second time around, I cut it down to 3 Tablespoons, or just short of 1/4 cup. Ginger Flavor is still there, but not so spicy.
I didn't scale the lemon/lime as much; it's pretty difficult to juice 1/8th of something. I just did 1/2 of each.