It's not about calling within certain time periods, i.e. every 20 minutes or 10 minutes. It is more about frequency in a short period of time.

The natural order of things is for the gobbler to wake up, gobble to let the hens know where he is, go to his strutting spot and wait for the hens to arrive. Calling him in is short circuiting the natural order.

So in regards to over calling.....Let's say that you have scouted and found an area that a gobbler is roosting. You ease in and try to get around 100 yards of him on the roost. At this point you make some very light calls...3-4 yelps that are very low with a few clucks. Tree yelps. If he answers you that first time let him simmer for a few minutes. After a short time give him a few more but keep them quiet. If he answers again just wait.

This is where the fun begins. Sometimes he comes to you, sometimes he doesn't. Once he is on the ground you can pick it up some. Start with some clucks. If he answers you are in good shape. A few yelps but still keep it easy. The key here is to not be too aggressive.

If he is really fired up you could fart and he would answer. If he isn't gobbling as hard you have to take it easier. Clucks and a seldom yelp will do. You don't want to call just to hear him gobble. What you really need to focus on is how hard the bird is gobbling or not. Overcalling has more to do with pushing him harder than he needs. If you call too hard he assumes you are a receptive hen that is about to come to him.