With Scout, coming to a whistle just sort of happened; it's more like if he sees me coming he makes his way to the gate (big walk/jog), and if he's too busy grazing to see me coming, I whistle, and he comes faster (brisk trot/lope) like he's upset that he didn't see me first, lol. I don't think I've had to walk out into the pasture to catch him since the first week I had him.
I'd say that it comes down to how your horse sees you; horses want to be near their herd leaders - that's a place of safety, etc. Having a solid foundation of general respect in groundwork and riding goes a long way toward establishing that perception of you in the horse.
Consistency is important, too. I bring mine in and feed them at 4 in the afternoon, and it doesn't take long for them to set their internal food-clocks to start looking for me at about that time anyway. I use the same whistle every time; I start with a low pitch, run up high, and back low, but as long as your horse can hear it, and it's consistent enough that he can associate it with you, it doesn't matter. Do my guys associate it with food? Probably, but they don't always get fed when I catch them. Sometimes they get groomed, sometimes we go for a ride, sometimes they just get to come in and stand in the shade in the barn (we unfortunately don't have much shade cover in the pasture).
Something else to keep in mind is that some horses are just so aloof that they aren't going to come at a whistle like that. Scout's a big baby, and loves nothing more than to be messed with and loved on. His "brother" Rio, on the other hand, is much more aloof in general, and will look up from his grazing and acknowledge my presence at the gate, and then put his head right back down and wait for me to come and get him.