Here's a copy of the actual thread you were referring to:
You can use the following to work backwards off listed IBO specs, this will put you pretty darn close to chrono:
IBO Specs - 30" draw/70lb draw weight and a 350 grain arrow.
For every inch of draw length you go down, you will lose 10 fps.
For every 5 # draw weight you go down you will lose 10 fps.
For every 3 grains of weight your arrow is over IBO (5 grains per pound) you will lose 1 fps.
For every 3 grains of weight you add to your string you will lose 1 fps.