This is where my Quest drum roaster and my P1 air roaster diverge. The first crack starts at 395F for both; but the second crack on the drum starts at 430F, while on the air roaster it starts at 445F. I am pretty sure this is an artifact of measurement, not a true difference in the way the beans develop.

Therefore, I use smell and sound to drop the roast. If I want a very light roast for cupping, I drop it the moment the aggressive, vinegary smells end (roughly at the end of the first crack). If I want my darkest roast for cappas, I wait for the 2nd crack to start; this coincides with obtrusive distillate smells. If I want something in between for regular brewing or espresso, I look for the first nice caramel smells for a lightish roast and the first distillate smells for a more darkish roast. To get to these different points in the same time requires dropping the heat more aggressively for the lighter finishes and less aggressively for the darker ones.