This is vexing and in progress and tbd. I have one recipe which involved boiling 2 T of shredded ginger with 1/2 c water and 1 c sugar. Let it cool, add 7 c water and 1/8 t yeast, let it ferment in a plastic bottle for 2 days, and there should be ginger ale in there then. Mine was flat and tasted more like Ginger Ade. Still good, but not Ale.
I had a hypothesis after tasting this one. I thought that since it had a great flavor but was flat, I could make the syrup and add it to sparkling water and have ginger ale. I found a recipe that was just that on the Whole Foods app.
This one called for 2 T diced ginger boiled with 2 c sugar and 2 c water. I thought that 2c sugar was overkill, so I reduced it to 1. Adding 1 liter of San Pelligrino to the cooled syrup produced weak ginger-hinted sugar water. So... I boiled about 4" of grated ginger in 2 c of water. I added it to the existing mix. It's still really sweet and therefore not as gingery as I want it to be because I need to dilute it with sparkling water to cut the sweet.
So- next time... I'm gonna boil about 5-6" of shredded ginger in as little water as possible 2-3 cups with 1/2 cup of sugar (I can't believe the original recipe said 2 cups!). Then dilute that with bubbles. That probably won't be done for awhile though because I have a ton of this sweet stuff to get rid of first!
I will get there.