There are two different types of syrup: two-parts-sugar-to-one-part-water, or equal parts. Essentially a lot of what you choose will come down to your palate, but we use both methods. Our basic sugar syrup is two-to-one, which is of course extremely sugary, but we use it very sparingly – a little bit goes a long way – so rather than adding more volume to a drink and changing the ratio of the liquid, we use just a very small amount.
When we make flavoured syrups, such as fruit syrups, we use a one-to-one syrup because the flavour then becomes part of the drink and you might need to use more of it. At home, I’d recommend making all syrups using a one-to-one ratio so that you don’t oversweeten things.