I very nearly never buy salad dressings any more, maybe once a year?
And usually I just wing it
I measure the olive oil, then throw in whatever vinegar/acid I feel like using, mustard for emulsification, and whatever seasonings I'm up for.
One of my favorites is olive oil/white wine vinegar/mustard/maple syrup (real maple, not the fake stuff)/salt+pepper over greens with apples and cheddar.
Or, olive oil/lime juice/honey/salt+pepper.
Tonight will just be olive oil, red wine vinegar, salt and pepper, mustard, and some Penzey's Tuscan Sunset blend for an Italian-ish dressing