So does the Pandemonium have an actual physical location within the Askellon sector? Or does it just encompass the entire region?

On page 322-323 the map of Askellon doesn't seem to show any definite location of the Pandemonium but does seem to have several illustrations that could be interpreted as being representative of it. Coupled with the fact that the Pandemonium is said to have " sentient" like qualities I would guess this is intended to let the GM use the Pandemonium in any way they see fit in the game.

my guess, to hammer home the point of how terrible it is, the Pandemonium is a sector-sized warp storm, and rather than flaring up for ages, all across its span, it is relatively calm in most locations, almost imperceptible, and just flares up, if you will, in various locations, all throughout the sector. In this way, any locale within Askellon can suffer from it, making the entire sector less popular, as opposed to just having that one area you should avoid. Not certain, mind, but it seems to be a sector-wide phenomenon to me, based on the book.

Also, it seems to be very bad at the sector's borders. So bad indeed that accessing the sector by Warp travel is very dangerous and even the black ships of the Inquisition are not daring to visit as often as they used to.

The Pandamonium is also somehow intrinsically linked to the activities of the ruling lords of Askellon. Not much more has been said beyond that, it would be great if this was explored further in future campaign supplements.