Studies show that the human eye is terrible at judging 2/3. We tend to overestimate. So always under-fill slightly if you’re eyeballing. You can add more, but you can’t take out too much flour.
For liquids, a wine glass (the standard 5-ounce one) is about 2/3 of a cup. So if you’re making a sauce and need a splash of wine, pour a glass, use what you need, and sip the rest. Chef’s treat.
