The general rule is to set your sensitivity as high as you can without causing the detector to become unstable,a nice quiet detector is more preferable than a noisy one because you have set the sensitivity too high.
As for finding iron,everybody does and a detector such as the 150 will pick up more than the latest complicated models.
I would set it like I set my c scope 3mx.......just discrim out small iron objects such as small nails and dig everything else.The adx150 is a cracking little machine and will find you loads of stuff,just stick at it and if you go over something the xp will find it for you.