touch the bottom of the chamber but will be in the middle of the beans. when you roast, watch the temperature. if you like what was roasted to 430 degrees, you know where to stop the next time.
use a digital scale. coffee bean varieties weigh differently. also, the beans lose moisture in the roasting process. for instance, 4 ounces of green coffee beans roast out to about 3.2 ounces. using a scale to weigh your beans will assure accuracy each time.
if you are roasting outside with a popcorn popper, the ambient temperature will affect the time necessary to reach that perfect roast. that is another good reason to use the 