Research carried out for the 2002 White Paper on future air transport development in the UK noted that a ratio of 1,000 employees per 1 million passengers was a commonly used 'rule of thumb'. The DoT funded detailed research by ECOTEC and Oxford Economic Forecasting (OEF) which suggested that whilst circumstances differ for individual airports as a broad measure it's reasonable to use. Things tend to get harder to evaluate as you move away from direct impacts of aviation.