Unless it has changed since I did the JMT.

Find a limb 15' off ground, preferably a down sloping branch.
(Note: Not too fat)
Throw rope over tree limb at least 10' from trunk.
Tie first bag (Equal weight to second bag) to end of rope.
Pull bag up to limb.
Stand on 'something' (If you can) and tie second bag to rope.
Secure excess rope to bag.
Push second bag up, as first bag comes down.
Make both bags equal.
15' to limb
8' up to second bag.

15' - 8' = 7'
7'/2 = 3 1/2' down from limb.
15' - 3 1/2' = 11 1/2' off the ground