Welcome to BCUK firstly
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Difficult question to answer, it depends greatly on how high you wear your web belt and how high you like a pack to ride.
I have used them before with the pack sitting on top of the webbing and it was comfortable enough, but I would suggest trying one first with your belt gear on.
One of the guys in your part of the country is bound to have one of these packs, try to get to one of the meets locally so you can try it out is my suggestion.
As I said in the earlier part of this thread they have changed a bit from my initial one that I bought (the one in the picture) so it may not be accurate to gauge by the picture whether it would be OK with a web belt.