Duomei Lemon Lime Soda  food flavour concentrate.
Water soluble.
Very yummy and profound lemon lime flavour around 3%, you might need more or less of it, so adjust the percentage uppon your tast buds.
 
You can start this one a bit higher around 2% and increase it up to 5-6%.(Standalone)
As an additive, start from 1% and increase it.