There is no exact formula for this, but there is no reason for the sling to be overly long and unpractical, no matter how long the staff is.  The sling on mine is a 27" poly/leather thing, slightly shortened when I adapted the loop to the staff. The staff I use is between 4 and 5 feet, but anything goes from 3 feet until as long as you can wield.
timann

I made three staff slings, one of which I kept.  The length of the sling almost didn't seem to matter.  Most of the oomph comes from the staff.  The first staff sling I made was relatively short, maybe four feet, with a very long sling on it.  That wouldn't hurl projectiles any farther than my hand slings, but it could hurl huge objects with ease.  It could toss fist sized rocks easily 80-100 yards.  I gave the second one away and honestly don't remember much about it.  The one I kept is about seven feet long and has a normal length sling on it.  This will toss egg sized rocks maybe 130 yards with very little effort, but the staff is too thin for anything bigger.  All of my staff slings were bamboo.  I used the simple sort of release.... the loop just fell off of the staff at the right time.  Somewhere on this site there's a graphic that shows how to adjust the length of the sling to control the angle of flight of your projectile.  If I can't find it I'll draw one for you.
  Staff slings are easy to make, almost effortless to use, and loads of fun.  I would still be making more but the neighborhood kids got the rest of my bamboo, then my supply temporarily dried up.  Hopefully I'll get more this summer.  There really should be a staff sling tutorial here.

  Some really great stuff here guys, I will have to try making one after I make the sling for S.I.T.H.  My war club is almost finished now, should be done tomorrow.  I also have to get ready to go to the farmers market this summer, I do portraits so I will have to set up a display, I also will be selling T-Shirts of my comic strip Chivaree Bay, found in the Quoddy Tides in Maine, as well as selling slings and war clubs I will be making.  Busy, busy.

The longer the string in a Staff Sling the more precipitously shot.

As a rule one can possibly say: The about half one should have the Sling long from the stick.
Then one can measure then stick well and score exactly on different distances.
I had here also pictures of my Staff Slings. If she cannot find again, however.

  I notice those old illustrations show a very short sling with a wide pouch, probably just an artists rendering but... and did you see the guy throwing that football sized rock??  That would take a little effort.

  How much space should there be between the tied on retention cord and the looped over release cord on a 4 foot staff sling?