Like everyone has said run the sensitivity as high as your are can.
Here is an air test for the Ace 250. All coins were silver except for the penny and nickel of course. S-8 is Sensitivity setting of 8, S-6 is Sensitivity of 6 and S-4 is Sensitivity of 4. Distances round down to the nearest 1/4". Notice that when the sensitivity is turned down so is the depth. It will be worst in the ground and depends on your soil conditions.
______________9x12 Coil - All Metal____________|____________6x9 Coil - All Metal
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_________S-8_________S-6_________S-4_______|______S-8_________S-6_________S-4
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1c_______10"_________9"__________7 1/2"_____|______9"__________8 1/4"_______6 1/2"
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5c_______10 1/4"______8 3/4"_______7"________|______9 1/2"_______8"__________6"
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10c______10"_________8"__________6 1/2"_____|______8 1/2"_______7 1/2"_______6"
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25c______11"_________9"__________7 1/2"_____|______9 1/2"_______8 1/4"_______7"
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50c______11 3/4"______9 3/4"_______8 1/4"_____|______10 3/4"______9 1/4"_______7 1/2"
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1.00______12 1/4"_____10 1/2"______8 1/2"______|______11"_________9 1/4"_______7 1/2"
What surprised me was the half dollar and dollar wasn't detected as much as I thought it should be. The air test was repeated twice with the same results. Also, the distances were the same for All Metal, Jewelry and Coin Modes.