I don't see a lemon tree listed on the guide for moisture. What is the moisture reading required for a lemon tree?

Citrus trees like an evenly moist soil, but not one that's soggy wet. So generally, a moisture reading of 3 or 4 is recommended. Sinking the meter into the soil 6 inches down will give you a good reading. And, as a double check, if the soil is dry 6 inches below the surface, your tree needs water.