The first step in tuning your drum speed is to measure the rpm of your drum. Even if the rpm set by the manufacturer should be optimal for your machine, it’s a good idea to verify that the rpm is correct on your roaster.
With the roaster turned off, stick a small piece of tape or a sticker to the front edge of the drum. If the door of your roaster doesn’t open far enough to allow you a view of the front edge of the drum, then check whether you can access the bearing at the back of the roaster and place a marker on that instead.