With a water meter which I uses as well once it goes 1 bar below the red its fine to water. 2 bars below the red I begin to see drooping which mean under watered.
I just do a fast test. Let them dry to the point of droop. I suggest checking them every half hour on the day you do this.
Once the hit droop point put the meter in the soil and see where it says you are. Now you know what point is underwatered and can guage off that meter to water right before that point.
Hope this helps.