Diet Pepsi syrup (the real deal) 2L jug
Pops are a 5:1 ratio. The colder and more carbonated your water the less syrup you'll use. Start with aprx 125ml or 3-4 pumps for a 1L bottle and adjust +/- from there. Experiment with the number of pumps to find the amount that suits your taste.
Thinking of shipping syrup? Max out the box with 8L of syrup for the best value.
Don't forget the syrup pump!