Iron has a tendency not to pinpoint dead center--it is usually a couple inches off center.
The "wiggle" works pretty good and the target will be under the "V" of the coil when the tone fades.
I use the "sizing" option because it doesn't keep detuning as you go over the target. For me, it gives a clearer indication where the target actually is.
If the target is on edge, alot of time you will hear a target tone on each side of the actual target while swinging parallel to the vertical target. The tone will go silent when you are directly over the target. If you swing perpendicular to the vertical target, the target will usually disappear (or sound extremely weak). This is something you might want to practice in a test garden to see what I'm talking about.
In the end, it does take some practice but it will get easier.