I never really worry about how deep coins are,with the massive array of coins around the world and also made of various different metals,what one person would call deep another may struggle to get the same depth.Also what settings works for one person does not mean it will work for others either.
Me personally i use just a 9'' coil in the UK,so our coinage can vary not only by say size but also say silver content as well,the deepest coin by far that i have found and i wont mention a depth because i dont like mentioning depths as detecting is not all about depth as such,the coin that i did find was a 1797 George111 Cartwheel penny at a pretty amazing depth but this is just because its a big coin and i mean very big and weighs 1 ounce infact a Cartwheel 2 pence is even bigger.So its not your everyday coin find but it still would come under the heading of deepest coin as it was legal tender.
One thing i do find frustrating is when someone starts a forum post like this asking what others have found and the setting etc but they dont start of with how deep they have found a coin and also the settings that they use,they expect other to provide information but dont seem to kick it off with any information themselves.
The bigger the coin the deeper the detector would go,so that is a big advantage for a start,also the ground conditions,in the UK our soil is pretty good,so what settings work for me certainly would not work for say someone in Culpeper Virginia as i understand that ground is pretty bad.
Just my personal input on this subject.
