Rather than reducing the other ingredients, it's easier to increase the volume of the raspberry syrup. Using your 2:1:1 recipe, you'll need 50% more raspberry syrup, i.e. 22.5ml (3/4oz) instead of 15ml (1/2oz). Comparing the finished cocktail to one made with a commercial syrup such as Monin, you'll find the vibrant colour from the homemade syrup more striking than the difference in flavour. I hope all the mums approve.