The amount of dwellings that they have used in their calculations seems very low. For comparison, the Vista apartments next to Fratton park (the blue building in this picture) contains 69 flats. If Fratton Park ever gets redeveloped I can guarantee that they'll squeeze more than 40-80 properties onto the site.
As another comparison, when they built on The Dell (which I would say is a smaller site) they built 202 apartments and 26 town houses.