The only trick with these devices is to learn when to stop the roast to get the darkness of roast and hence the taste you prefer. The longer the green beans roast, the darker the color and the deeper and less bright the taste. Because batches of green coffee beans differ from one another in density and moisture content, it is impossible for the manufacturers (or for me) to specify exact roasting settings or times.
One way to approach timing the roast is to roast a batch of beans on the "Medium" setting, brew the result, and fine-tune the setting from there for subsequent batches, based on whether you want a sweeter, richer, more pungent taste (set the timer for a longer roast) or a brighter, drier, brisker taste (set the timer for a shorter roast).
When you arrive at a setting that produces a roast that satisfies you, make a note of it. You should be able to set the timer to the same point and obtain similar results for subsequent roast sessions. However, every time you buy a new batch of green beans you probably will need to experiment again, modifying the setting slightly to produce your preferred roast color and taste. Decaffeinated beans and aged beans, both of which start out brown and roast either very quickly (decaffeinated beans) or very slowly (aged beans), may require particularly watchful experiment to obtain a satisfactory roast.