10 to 1 would be a bit much. 5 to 1 indicates the syrup is already thinned out, which is a possibility if the denser syrup doesn't mix readily with the fizzy without stirring.

But then, I'd figure there was purchase-density syrup, and then pre-make which is ready to dispense with fizz water, and a clerk is tasked with diluting one to make the other.

That's the way it'd be if I were writing a fast food sim.