I use an XLT, and the usual range for (shallow, 1-3" deep) indian heads is from 58 to about 64. Generally speaking, if they are near new and aren't corroded, they are closer to 70. If they are very worn and/or corroded, the number is lower. The highest I have found was probably a +68 and the lowest I recall was just a +39 (worn out and thin 1869). Now if they are deep, like 5-7" deep, the signal can go very high on the VDI scale, and they may give a broken sound like a rusty bottlecap. So if you want those really deep ones, dign any questionable signals at a depth of greater than 4 inches. I bet you'll find a few deep ones that gave crazy readings on the meter.
The copper-nickel indians minted from 1859-1864 are made of a different alloy and read out as pulltabs- about +31-32; and so do flying eagle cents. Those will be a real pain to identify in an old park.