I think part of the reason the chute deployed so low is my launchpad setup.
This 1/4" x 48" rod is at quite an angle. I normally use it for RC Rocket Glider launches at an RC airfield.
The safety rules at the airfield stipulate that you do NOT let your aircraft (or rocket glider) cross the flightline. So, I am launching at a fair angle (5 to 10 degrees?) to get the rocket glider out over the runway.
A more 'straight up' launch rod setup would have got the Nell higher. Still, I think I will stick with B6-2 or C6-3 motors in the future.