We can’t really advise on a target humidity level, as we find too may household devices do not read the levels reliably.The best gauge is that oft repeated recommendation “a regularly used modern frost free kitchen refrigerator located in a room temperature environment.”
You’re right that outside environment can affect humidity, as can the contents of the fridge, However, if in regular use, most normally functioning (aka this without frost encased freezers) can keep up with the humidity management.
A piece of meat that has been handled prior to applying the UMAi Dry® membrane is almost guaranteed to be “inoculated” with mold spores from the working surface. These can grow on the surface and do not pose a risk unless they start to turn color. In the photos, if appears that any apparent growth has not gone green or black. If it did, we would recommend you immediately abort the drying process.
You will accumulate more familiarity with the process as time goes by. The first time is always fraught with questions and concern.
The best test is to watch for blueish or greenish tinge and–most of all–to sniff the surface of the meat for a nutty “blue-cheese-y” forest-y funk. It is distinct from the scent of rotten meat, but not the smell of roses. You will learn to discern the difference and enjoy the process as you gain more experience.