An investigation is presented of the relationships between various greenhouse lighting performance parameters and the associated daylight percentile (commonly known as "percentile exceedence"). Analyses were carried out on the National Solar Radiation Database (NSRDB) from the National Renewable Energy Laboratory to find the most suitable percentile value to correspond to the lowest running average Daily Daylight Integral (DDI) for a site, and to the lowest monthly average. The modeled production of hydroponic lettuce was also analysed at different percentiles. Results indicate that the most suitable percentile for a running average criterion is equal to P = 101.0 - 0.4779 L, where P equals the percentile and L equals the length of the running average in days. The minimum monthly DLI corresponds to a percentile of 83.7%. Percentiles associated with the modelled growth of hydroponic lettuce varied from 86 to 99% depending on required mass of the crop. These results can be used by designers to select the most appropriate design percentile for a facility, and in turn to design an optimum supplementary lighting system for a given greenhouse.