I would suggest that where you need to use a wedge or block to level across the van it's more likely to be on the awning side.
As its nicer looking at a slope falling away from you that the other way?
Thus the awning length needs to be the normal level ground "A" dimension plus twice the "block" height.
Also its much easier to "lose" a bit too much length by splaying out the side close to the van, than live with a short awning.
So for me it's a simple decision, a touch too big is the better option if the right size is not available.
Cheers John