I now put the items much further apart (2 feet) as there's so many large coils now that retain sensitivity or detect sideways wider than the apparent coil size.
Put a variety of good and bad items in by all means but also put in a row of the same type of coin at 5,6,7,8 to 12 inches. I use pure copper that are about one inch wide. You can then do a real test of detectors that offer standard (Tesoro beep and dig) type discrimination and tone discrimination. With my T2 for instance, standard discrimination gives a gain in depth of one to two inches over the Teknetics tone option. The coin will still be detected but the tone audio indicates the good target as iron.
I have the test beds a few feet out from a hedge bottom where the markers are. Keep a note/draw a plan of whats what. When you get a new coil or detector test it out in its different modes and note that down so you can repeat the test every few months. This will indicate if something is starting to fail before you get to the end of a year and wonder why though your copper count is similar to the year before your silver finds are way down.