Certainly not as elaborate as Steve's but I planted a 3D version of Monte's Nail Board with dug square nails @ 1-4" around a Cu penny @ 6". It's a tough target, but not unrealistic for some of my sites. It has really helped me to see the need to slow way down and massage each hit, It has also shown what each size coil can do and how different frequencies react with the ferrous and non ferrous. Without this 3D set up I would have just gone with the conventional wisdom that a higher frequency is better in iron, Now I know how 20kHz compares with 14kHz and 5kHz. High gain vs. moderate gain, I was surprised by the clarity 5 kHz gives the tones, but you have to go slow and keep good overlap on any frequency. Now I know another way to help unlock a keeper out of the iron. 5 is not as quick as 20 but it adds it's own attributes if you let it.