I have only 3 roasts under my belt using a popcorn popper (or any other method, for that matter). My beans (Sumata Mandheling) will reach second crack anywhere from 6.5 to 7 minutes. So your times seem right to me. However, total time, does not seem like a consistent way to roast.
I have been told that the time after first crack roll is a better way to time beans. Not total time. First crack roll is when you start to hear several cracks in a row.
Still, I am having concerns that I always have a first crack roll.
The popcorn poppers can roast quickly so it makes it hard.
I also struggle getting an even roast with my popcorn popper. The advice given to me was to shake it a lot. I will try that when I get to my fourth roast.
I think looking at the beans might be one of the better ways to tell roast level. Also, weigh the beans before and after. The percentage difference will be an indicator of roast level. There is a chart on the web if you search for it. After you do more and more roasts, you should start to get the hang of how done the beans should be. BTW each bean tastes best at its own unique roast level. Take lots of notes while roasting.
Listen to me. Three roasts and I am an expert, lol NOT!!! (do the kids still say that?).
Good luck with your roasting!!!