It’s really personal preference. You should for sure reach the First Crack. Less then that will still have some grain-like tastes. On the East Coast, this is a pretty popular roast level and they’d tell you that’s how it's done. Just into the first crack.
As you keep going the sugars in the coffee will begin to caramelize like Cracker Jacks or Creme Brule. As you go further you trade the sweetness and varietal nature of the coffee for the taste of those sugars caramelized. By the time you see smoke you are burning sugars and taking on that roast flavor of West Coast coffee. They’ll tell you that’s how it’s done.
So… at one end you have a sweeter coffee with more range, at the other end, you have a darker but not so sweet coffee with a narrow range. You trade the floral/fruity regional notes of the coffee for the taste of the process. All in subtle degrees.
Kind of like the color wheel, different regions have their qualities. India’s Monsooned Malabar, for instance, is ALL about body, but not a lot of flavor, so it’s good to use as a blender coffee for an effect. Sumatra leans into earthiness like good soil, in the extreme, kind of funky. North Africa tends to be bright and sparkly and complex with berry/fruit. Yemen is fruity, plus winey too. Island coffees like Jamaica and Kauai tend to be a clean, round cup (not spinning off in any one direction) and can have some nuttiness to them.
In different kinds of coffee along the way are notes of chocolate, flowers, tropical fruits, berries, etc. And you have a lot of control when roasting what qualities to bring up- a fast roast will tend to be bright and sparkly, a slow roast will tend to have more body, kind of like using an equalizer on a stereo. Different qualities can be controlled. The darker you go the more the subtle flavors are replaced by the process itself. That roasted flavor.
Processing matters. Washed process coffees (dried without the fruit on them) tend to be simpler and cleaner, like a simpler tune. Natural process (dried with the fruit on) tend to be wilder, more complex, more fruity. More on processing coming up.
How long you let the coffee ‘rest' after roasting, a few days or so is good, will make a big difference in the cup too. And degree of grind, and temperature of the water… all simple stuff, but very hard to quantify as an absolute recipe. HOW you roast will depend on the end result you're after with grind etc factored in.
Grossly generalizing- A simpler coffees Kauai, Costa Rica, Guatemala are fine with a shorter resting period 24 hours or so. More complex coffee like North Africa or many espresso blends benefit from a longer resting period.
"How do I roast this” is one of those questions that’s hard to answer, because it depends entirely on you. Like, How do I play this piano? :) How should I cook this steak? Or, how do I use this tube of paint? It depends… the raw materials have nature but can also do ALL kinds of things that are not right or wrong.
Best thing to do is bracket a few roasts and see what you like, then keep going in that direction until you stop liking it. Go hot and fast light and hot and fast dark. Go cooler and longer both light and dark. Reach first crack fast then drop the temperature to extend the time to the second crack. Or shorten that period. Etc etc etc. Jot down what you did, then only change one factor at a time until you see a pattern.
You will ALSO find that your tastes change over time. You'll have favorite beans, then get a new favorite bean. You’ll be into one kind of roasting style then discover a new roasting style. It’s like talking about whats best in art or music. Except for some broad rules of thumb, it's really hard to say what’s the correct way.
Roasting and brewing coffee is also a lot like live theater. Even with the effort to be spot on, it will be a bit different every time. Like literature or paintings, while the art is unchanged, it’s different because you are different.
Best practice with roasting a new coffee is to start with what’s worked in the past, bracket and pay attention to the difference. Use your eyes, ears, and nose. Try to only change one factor at a time. There is no absolute recipe for roasting coffee, only general rules of thumb, there are just too many factors, too many individual preferences.
Next time… Degassing