"Tests conducted in the NASA and Wright Field vertical wind tunnels determined that for vertical descending bodies, the parachute should be ejected to a distance equivalent to more than four times–and preferably six times–the forebody diameter, into good airflow behind the forebody." (Knacke, 5-21)
What Knacke has to say on webbing length for pilot chutes that are permanently attached to reefed main chutes: "The distance between the vent of the main parachute and the leading edge of the pilot chute, Lb, should be long enough to permit inflation of the pilot chute in good airflow behind the vehicle... The pilot chute bridle also must be long enough to permit full inflation of all parachutes in the cluster, allowing a space equal to about 10% of the inflated canopy diameter between the individual canopies. The pilot chute should stay inflated during the reefed canopy opening, but should collapse after full canopy inflation... Several operational clusters with permanently attached pilot chutes used a pilot chute bridle line length of Lb equal to 0.4 to 0.55% of the diameter of the main parachute." (Knacke, 6-40 - 6-41)
Resources
T.W. Knacke, Parachute Recovery Systems: Design Manual