I leave 1 plug out thru the entire incubation process & hatch. You can get a very cheap hygrometer (humidity monitor) from Wal-Mart for less than $5. It will give you an approximate humidity level, which is all I ever worry about. Humidity does NOT have to be exact, just close to your desired average most of the time. I tend to do what is called a "dry hatch" which means you do not add water until lockdown & even then only if absolutely necessary. Humidity will automatically spike drastically when the 1st baby begins to hatch so usually no water is needed as long as you leave the bator closed during hatch. If humidity goes to at least 50% when the 1st baby pips you don't need to add water. A lot of people use a straw & syringe thru the open vent hole to add water during lockdown.