I have limited experience using a decoy in a calling routine. I have used them quite a bit more while setting in a tree stand.
When calling I have had elk look over my decoy, ignore it, or walk back and forth, then leave. I have heard that flashing a decoy works on occasion and moving it also helps.
A stationary decoy set up at a water hole gives mixed results. Some elk see the decoy and immediately turn and leave. Some see the decoy and hang back then leave. Others will cautiously approach, get a drink while nervously looking up at the decoy about every two seconds. When they finish they leave ASAP. Other elk will come right in, go over and smell the decoy a while, get a drink then go back and sniff at the decoy, before they casually leave.
I do not use decoys with a tree stand set-up anymore. I would use one when calling. I would try not to give the bull a long look at the decoy, but enough to convince him he is seeing the elk he heard. I would set up ten to twenty yards in front of the decoy. Also try to rig it so it can be pulled away or down when he was not looking. The difference between the tree stand set-up and calling is that a bull does not expect to come upon another elk at the water hole, but he does expect to see the elk he heard from a calling routine.