If I've got this right, what Grendel is saying that the average Sixfields attendance represents supporters that might have just taken in an occasional game. On that basis he suggests it should be inflated to a larger figure.
However, he completely disregards the fact that every other average attendance in the league, including ours when we were at the Ricoh, similarly takes no account of supporters that attend on an occasional basis. Surely on that basis those figures should also be inflated proportionally.
I'd respect that the individuals who go to Sixfields have made their decision, and they should be free to do so (even though personally I'd rather they didn't). However, I'm afraid I've no respect for the flawed logic here, trying to proportionally inflate the Sixfields attendance into something it isn't.
In raw numbers, using Grendel's assumptions, if there's an "extra" 600 occasional supporters at Sixfields watching the club, then that represents an additional 2500 occasional supporters who are being denied the chance to watch them at the Ricoh.