I'm kinda having same problem as you atm. What helped was getting a kitchen scale to weigh your medium bag dry then fully saturated and you deduce the water content for a small plant then you can do proper drybacks and maintenane shots as well as how much she transpires once they pick it up. Note that a standard 5Kg scale won't take more than a 2 gallon and you will probably have to make a stand to properly use the scale so check the scale capacity for your medium but if you can calculate water content you'll do better in a smaller bag.🙌
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