Awning Sizes

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Hi all, just bought a lovely Jubilee Ace Evnvoy caravan, and need to buy an awning for it. Got a bit confused as to the measuring of caravan for awning, how exact do your measurements have to be, if its a few centimetres longer, does it really matter? Some good used ones available but need to know if i have any leeway with fitting? Any advice very welcome, thanks, Bob

P,S. States 890 ground to ground, but iam looking at an Isabella 930, could i use that?