There are many factors influencing water requirements, e.g. soil properties, pot size, temperature, air humidity, and of course the plant. The more of them you know about and the more of them you can monitor and control, the more precise you can calculate water needs of the plant. Basic will be weighing of pots, soil, and water before sowing and monitoring weight throughout the growth. Then you know how much water goes in and out. The more constant the environment around the plants is, the better measured water use correlates to plant activity. If there are strong fluctuations in VPD, measured data get incertain, because there will be changing evaporation from the soil surface. Beyond monitoring weight, there are much more sophisticated methods available, as you may find in litereature on water use of plants.

There are many factors influencing water requirements, e.g. soil properties, pot size, temperature, air humidity, and of course the plant. The more of them you know about and the more of them you can monitor and control, the more precise you can calculate water needs of the plant. Basic will be weighing of pots, soil, and water before sowing and monitoring weight throughout the growth. Then you know how much water goes in and out. The more constant the environment around the plants is, the better measured water use correlates to plant activity. If there are strong fluctuations in VPD, measured data get incertain, because there will be changing evaporation from the soil surface. Beyond monitoring weight, there are much more sophisticated methods available, as you may find in litereature on water use of plants.

We used TDR probes to control the soil water content and to maintain 60% of water holding capacity.

I used pot growing trees and measured water requirement by weighing automatically representative of the trees. No doubt fhis is the most accurate measurement as it integrates all factors affecting water use by the plant and by evaporation from the soil ( if you leave the soil exposed). I irrigated once during the night in access and measured the field capacity after drainage before dawn. The water consumption was calculated from dawn to dawn. Measurements were taken continuously. You can monitor the increase in plant organs daily ( the Weight change at field capacity). I could weigh also the weight of dew deposited on the plants.

I use TDR probes at different depth to controll the SWC.