Eyesight can range from far across a field or 10 yards away in a thicket. If it is coming in there is no need to keep calling. If it stops or looks and then does a slow spin give a soft yelp or cluck may convince him to come on in. In Nature the hen goes to the gobbler, not the other way around. He is just trying to beat another gobbler for mating the hen or is out of hens when he gets to coming to the hen calls.
More times than not he knows right where you are and any deviance from coming straight in can indicate suspicion on the part of the bird. There are no set rules to calling as such. When in heavily hunted areas err on the side of infrequent calling and use clucks, purrs, soft yelps for better success.
