Buy a cheap digital kitchen scale, find the sg of all your ingredients and convert required volumes to g or oz and rewrite the recipe.
Then use the add 'n' weigh facility to make the perfect cocktail combo.
Also saves on the washing up as you can use your shaker or cocktail glass as the container.
Hell, moving on, you could turn this into a microprocessor controlled cocktail mixing gizmo to deliver any dialled in drink! Must be a huge market out their just gagging for this ....
Andy