It depends on terrain of course, but if the vegetation is no more than a couple of feet high and the ground is fairly level, then I tend to think of the next wind gradient starting between 15-20 feet above ground level in relatively average conditions.
The other thing is this, the faster the wind is blowing the more these layers compress. The boundary layer next to the ground might be 25 ft high at 0-3 mph, 15 ft high at 5-8 mph, and 5 ft high at 10+mph.
If I am shooting far enough to pass into the next highest wind gradient, I add 30% to the wind for the time the bullet will be in that gradient. That sounds technical, but when boiled down it just means that most of the time I need to add and additional 1 or 2 tenths to my wind call.
The only time it really comes into play is at ELR distances. The bullet might spend half it's flight time in the next higher gradient.
