It doesn't need reducing, it's already the correct size for your van. 10 to 12 cm either end will make no difference at all. Fractionally more or less mud wall laying on the grass. Please don't waste your money, I'm sure that the handbook is absolutely correct....Mick
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I agree with the above, awnings are made in 25cm/250mm or in English, 10inch jumps, this is so it fits many different vans with 'A' measurements between the two sizes stated, the awnings 'A' measurement or actual length is the bigger one, in this case 950cm.
Any slack is divided by two as it's shared at front and back, pulling this out so it's taught at the van is the first thing too do when putting up, the wings are then there to seal it back too the van.
The advantage of having it bigger than required for a level pitch is if the pitch slopes away from the van, this increases the 'A' measurement by double the amount you need to level the van on the awning side and a longer awning will still fit.
I would always suggest having it a little bigger than required. I used to have an adria already 542dt that required an awning of 1000cm. Got a dorema size 14 975-1000 it fitted but if the caravan wasn't on a 100% flat we would struggle to peg it down to the point I had to buy longer peg down ladders. I have since changed my caravan which now needs an awning of 1068 so have had it extended. Not tried it yet but hope there is a little more slack this time.