It doesn't matter how far away from your COL you have your transmitter, as long as all of the COL city limits fall within your Grade A contour. Being that Philly is oh so close to Wilmington, and height means "everything" to FM, I'd build as close to Philly as I could, while maintaining my Grade A into Wilmington for the sake of getting into the Philly Book, so I could charge near Philly rates.