If there is a shallow dish for the water ,top it up from the start with warm water .You could also give a little spary one or twice a day ,with tepid water from a mister type spray thingy..... In my home made incubator the water tray was directly above the heat supply , as in the old paraffin greenhouse heaters !, so had no problems with it . I never had a humidity tester either ! just a thermometer . The main things you don't want are totally dry or dripping wet , somewhere inbetween would be fine . I personally would just use it as you have it now 33% plus the odd spray as needed, it should work out fine. Hope that helps
Russ