Help! What size Awning ?

Laffy

Hi All,

I'm new to Touring & was just looking at Awnings!

 I have found out that the A Measurement is 1085cm.  The bit where I'm now getting confused is do I need to get a Size 18 which states 1075-1100 on Dorema website or when I put in make & model of my Caravan on Isabella website it states I need an 1100cm,
assuming this would be 1100-1125. Please help as I'm saving the money for one & don't want to blow it & get the wrong size?

Many Thanks