Caravan Awning Size Swift Challenger

Hi, I would be so grateful if somebody could help me find out the size awning required for my caravan. I am aware of the A to B measurement but the problem is Im hoping to purchase a new awning before next going down to the caravan as it is sited. The caravan is a 1986(i think) Swift Challenger 600/4. its approx 22ft length, I have tried to find out through Swift but they dont have the information as it is an old van! So i would appreciate if anyone else could help me, many many thanks in advance.

I think the awning size you need is 998cm. However, if you ask the dealer or manufacturer from where you are buying the awning from, they should be able to confirm to you the size you need from their data base.