Humidity guages IMO are not very accurate and not necessary for tarantulas. Spritzing the substrate, webbing or sides of the enclosures, lightly should do fine. You do not want it so damp that condensation forms, as that can get moldy.
Hi, you don't need a monitor or somekind of, if you have the Slings into a room, the temperature inside the room is okay & for humidity only put some water drops in one side of the enclosure twice each week. I guess that is enough.
The amount of ventilation, room humidity, and where in the world you come from should determine when you water. For most sling let the substrate barely dry out, just a little.
You will have to make a determination when you should water. unfortunately every one believes that their home setup is like everyone else. If you give alot of ventilation, have a high temperature, and live in a low humidity area then you may need to water more. If you live in a high humidity, low temperature, and well ventilated container areas, then you need to water less.