The way I did it was cord through the awning channel from ground to ground either side and measure the cord take that measurement to awning supplyer they will give the right size. However most have awning/caravan charts and may tell you to save messing around with a cord.
Or if still trading give them a ring.
Elddis Caravans
Delves Lane
Consett
Durham
DH8 7LG
01207 503477
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.