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The Seemingly never-ending Question of exit separation 

So How do I Know how long to wait between groups ?

Here is the Short Answer:
Uppers in mph > Seconds
0   =   8
10 =   9
20 = 10
30 = 11
40 = 13
50 = 16
60 = 21
70 = 28
80 = 43
90 = 97
95 = 257
96 = 383
97 = 751

You will hear 45-degree angle. THAT DOES NOT WORK.
Exit Separation Revisited - By Bill von Novak

You will hear look at the ground. YOU CAN'T JUDGE IT.

For a Great visual explanation got to : John Kallend's freefall simulation program

For more details & discussion check out : Fundamentals of Freefall Drift and Separation

Another Look at Descent Kinematics : http://www.koyn.com/clouddancer/articles/jmprun98.html

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