Rule of thumb seems to be 100 yards for a distance from roosted birds. Closer would work too. Depends if you can get there without spooking the birds. Some will say NEVER call to a Tom in the tree. I usually can't resist, but when I do call I just do a few very light tree yelps, just to let him know I'm there. I do that if there are or aren't hens there already. If he's with hens I just let it play out for awhile. If the hens are vocal I will be too. If things are quiet I stay soft with my calling. There are so many other factors involved. One of the things I've learned if the Tom is with hens and gobbling his brains out, keep a eye out for the ones that aren't gobbling. Subordinate Toms will often slip in to your calling. They won't go to the group if they got beat-up by the boss in a earlier meeting. Two years ago on opening day all the gobbling was to my right, I filled with a quiet one that came in from my left. Never heard a peep out of him.
For what it's worth!