Dont use measurements, use your eye and a cocktail stick. When u water you will notice your plant sort of stands up to attention. Looks percky. When u water too much it will droop alittle, and if you dont water it will droop. Wait will its droopin abit and water.
The cocktail sticks helps to show you the moisture of the soil, stick it in and if there is dirt on it there is no need to water, if there is no soil stuck to it give it some water. Basically when the top 2-3" dry you should water, but not loads. I aim to keep my plant looking percky. If it starts to droop ill give it some water, in 30 mins youll see the difference