How to Care for Agave Utahensis 

I dont have means of weighing accurately to work out when to water, my plants are in all sorts of sizes and shapes of plants and I permanently agonise about when the plant has dried out sufficiently to water again as I want to maximise growth during our short summers. I bought a cheap moisture meter but dont trust it. Is there any other way to decide when the soil is dried out enough. I am just guesstimating based on time elapsed, weather and weight of pot, but the weather varies so much from day to day in the Uk where I live it is challenging!