By weight.
What matters are the purpose and the conditions of the feeding which guides the mix ratio. Thick vs thin syrup matters. Exact ratio does not.
Thin syrup is stimulating; it will drive brooding and wax building. In cool weather, thin syrup does not get used directly and is difficult for the bees to dry and process properly for storage. That causes lots of excess moisture in the hive leading to mold, fermentation, and drunk bees with bellyaches. By contrast in warm weather thin syrup creates alot of activity and seemingly cleaner healthier hives.
Thick syrup packs on the stores in cool weather to bulk up weight for winter or long dearth periods.
So yeah .. it depends ;) , perhaps that is why it seems complicated.