On an average summer day (want is that!) about 3mm moisture (rainfall equivalent) is lost via evaporation and transpiration. But more if windy and sunny, less if dull so yes, you can do your head in trying to work it our in variable weather.
Soil moisture is indeed a great thing to check, also bear in mind plants’ needs eg onions need v little now, celeriac a lot, salads little and more often. If not sure about watering, I would not.