If they have any sense, to test the water, say 250 looks about right for the first bash. Then see where they stand, rather than set themselves up for any backtracking.
As mentioned before, the novelty of our first PL game away combined with new season enthusiasm and the wish for some points...
I would imagine that Leics and Watford will sell pretty quickly, a combination of fans wanting to go obviously with the addition of other 'borderline' pointers looking to top-up for later or more significant matches.