I use more than one hygrometer. Just filling the trays as recommended by the manufacturer will get the humidity close, but the ambient temperatures, humidity, and depending on how many vent holes you have or don't have open in the incubator will also effect humidity levels. Don't know how much a difference it makes, but I always set the temp at 99.5 on air circulated incubators. Some also recommend lowering the temp one degree for last 3 days. There's more info to read under the "Incubation" section too.
Good Luck!!