Personally I would always go for the slightly bigger size. Reasons as follows
The "A-Frame" measurement given by the maker is measured in a factory on a concrete floor that is perfectly flat and level, as is the caravan at the time of measurement. Hopefully the van weight was correct at the time of measurement as well as tyre size and pressures, all of which will influence the accuracy of that measurement.
In real life the surface you are on is seldom level. If the ground "falls away" side-to-side and you have had to use a wedge to get level, then you have, effectively, increased the A measurement by raising the wheel off the ground. If the ground slopes fore/aft by more than a few inches front/rear the same thing occurs. Likewise any dips/hollows where you are about to peg down can have major effects !
It is very easy to angle the ends outwards to take up a small amount of "excess" material if the awning is slightly long, it is very difficult to peg the ends down securely if they don't reach the ground when the awning is slightly short ... (ok... elastic extenders can be used but I don't trust them)
Just my view .. but in 30 years of vanning I've had both long and short awnings ... and long are far easier to accommodate .. every time !
HTH