I’m terrible in that I don’t pay attention to measuring when I’m making salad dressing. I make whatever fits into my glass jar that I’m using to hold it.
My go to is usually a sherry shallot vinaigrette type of dressing, but I’ll sub sherry for various fruit vinegars or even champagne vinegar if I have it. I always start with shallots (whatever bulb(s) are leftover) and mince and then I start to pour vinegar and a drop of mustard in for the base - how much depends on how big is my jar. 
. Then I whisk in oil until it tastes good. I start off with half of what I want, and usually will add both vinegar and oil until the balance is there, and then salt and pepper at the end. I also prefer dressings that are more tart and also more liquid-like, than thick ones.
