Base ENY on the number of Fields a country owns, not how many people are online. That way if say the Knights get a boost of players during prime time the ENY wouldn't go up unless we actually owned more of the map.
If the whole idea of ENY is to slow down land grabs, and help balance the game. Then tie it to fields, people are just too volatile, coming & going. Sitting in the tower drinking coffee or arguing with the missus.
That way ONLY the country that owns more of its share of the map gets punished. That way a large squadron could actually log into the Early or Midwar arena and be able to fly something other than the Val. Then if they started taking fields the Eny would GRADUALLY increase with each field taken.
No wild swings, but a much smoother control.