The measurement you have will be for a full awning. That is where the thick edging (known as Kador) sewn on the awning goes all the way through the awning channel from the ground on the front to the ground on the back. There are a few makers of air awning that do a full awning, Dorema is one. However most air awnings are “porch” awnings where the Kador only sits in the straight piece of rail parallel to the roof. Your A measurement here is no good to you and the only way to work it out is with the van and a tape measure in the same place at the same time. You also need to look at whether he walls of the porch awning cross windows or lockers.
Many makers in this field; Kampa, Vango, Isabella, Walker, as well as budget makes.
Attwools have lots of videos on you tube that are worth preusing.
Enjoy the hunt.
mel