Like you i am still relatively new to using a Corretto. But after quite a few goes now i have a a better understanding of it. Have you got a top on your bread maker while you roast? i say this as i have tried both methods with and without a top and must say i get alot more consistent results with a top on the bread maker. My main quesion to you is what type of heat gun are you using? is a 2000 w heat gun? Does the heat gun have variable speeds from 50-600 degrees on it?
To answer your question about first crack as alot of people seem to be saying in this forum there is no right or wrong as long as we get the result we want at the end. However i believe in a corretto first crack should be in the vicinity of 7-11 mins. However dont take my word on that. There are a lot of people on here with endless information.
Good luck and happy roasting :)