Jono Targett

This is a terrible idea. When measuring ingredients for cocktails you're going to be dealing with fluids of vastly different densities. A sugar syrup will be 30% more dense than water if not more, whereas alcohol is only 80% as dense as water. Using an estimate of 1 g = 1 mL across the board will result in badly mis-measured cocktails.

This idea would be fine if the recipes you are using were written for weights, but you can't assume that weight and volume are the same thing and expect good results.