Hello! super sorry for the delay, we have been traveling and I missed this, my sincere apologies, if the flatness is due to lack of exposure, get the shape correct smaller first, it really is the way, as sen in Developing the Outrun,
however, 600-900 yards is what I’d consider too far for a pro novice dog. You mention your open dog and 2 videos that may help accelerate developing correctness at the top (if your dog is tight) include:
A Cone Exercise for Widening the Outrun,
and the above exercise doesn’t apply, we show another method in our Course, The Outrun,
Please be in touch if you need anything else and hope that helps!