How to achieve separation?

1. Look down and watch until 1,200 ft have passed. (Skratch’s method).

2. Count to 5, then jump (Otter covers about 750 ft in this time on jump run at 13,000ft, no wind)

3. Wait for time taken to cover 1,200ft across ground (groundspeed method)

Question: Do these methods work?

In practice, what do skydivers do to ensure separation between groups. Several methods are in use: One way is just to count to 5 and then go (the problem with this is fairly obvious). Another is to look down when the group before you exits, and wait until the spot vertically below has moved 1,200ft across the ground before your group exits.  A third is to use the groundspeed of the plane to calculate how long it will take to cover 1,200ft across the ground, and exit when that time has elapsed. Do these ground-referenced methods work, and if they have shortcomings, what are they?