When using natural ventilation, calculate humidification load based on expected air changes per hour within the conditioned space. When using mechanical ventilation, calculate humidification load based on the amount of outside air entering the conditioned space.
A quick and accurate way to calculate load correctly is to use sizing and selection software distributed by most humidification system manufacturers. Software is especially helpful when calculating humidification load for buildings using economizer control, which require calculating humidification load based on maximum outdoor air intake at given temperature and rh values.