If the differential is tending to 0 then it means that not only the soil is saturated, but the plant is as well. This is _quite_ a basic concept, since you do not want a fully saturated soil all of the time.

A proper biological approach is to estimate how much water exactly the plant needs per unit of time, and then make sure the differential is always exactly that amount. This is not an exact science, especially when you're only working with a single plant and your conditions are probably far from ideal.

This approach would easily take into account other hidden variables, such as the rate of evaporation of water from the soil that depends on the ambient temperature. It also scales to hydroponics.