I agree - another good way to tell is to lift the pots just after you water to get a good feel of how 'heavy' it feels with full watering...then lift it like that every so often so you can get a feel for how heavy it is when it's dry enough to water again. This method is helpful since you'll notice that the plants will actually use more water at different times during the grow cycle, for instance when you're flowering, they may actually use water/food faster than when vegging. Some strains will actually perform better with fewer waterings as they may prefer it to be a 'drier' climate....in that case, watch for the leaves to start to drop a little before giving more water.... just my 2 cents. =)