First off, welcome to the hobby and the X-TERRA forum. If I were to offer one additional bit of advice, it would be that learning your detector is a journey, not a destination. There isn't a time I go hunting that I am not able to learn more about how (and why) my detector behaves like it does.
No pun intended, but in regard to what to expect your "mineralization" to be for "black sandy loam" covers a lot of ground. On the surface (another pun?), black sandy loam tells me that there is a lot of organic material in that soil. Although organic material is not magnetic, it can have a great influence on the conductivity of the soil. So many other factors come into play..... how deep is this top soil? What is the mixture of the other components? Is the surface water in the area highly mineralized? At the risk of sounding evasive, I'd suggest the best way to determine how mineralized your site is will be to hook up the coil and see what it takes to neutralize it's effects. Black sandy loam in my neck of the woods would represent moderately mineralized soil as the runoff from adjoining hillsides is not loaded with minerals. Most of the changes I notice in mineralization are due to generations of fertilizer application on the row crop land. On the other hand, if the sites you hunt have granite in the hills or the region has iron deposits, you will be facing much more highly mineralized sites, even though the top soil is considered black sandy loam. Again, sorry I can't predict what you'll encounter. But if you give us a general idea as to where you live, someone else on here might be able to share their experiences from that area.
Thanks again for the kudos. I look forward to hearing what you find out when you put the coil to the soil. HH Randy