I do this too. I also use a food scale to weigh out the popcorn within about a gram. I figured out the number of seconds required, so I no longer have to listen.
The process is surprisingly reproducible. I get very few unpopped kernels, usually around 5 of them.
66g of popcorn (that's two servings, according to the label).
2 minutes and 8 seconds.
After cooking, add oil/butter and popcorn salt, then toss. Popcorn salt likes to fall to the bottom, so after tossing, I put on the popper lid, carefully flip it upside down, and shake.
Also, I use a silicone popper, not a paper bag. (Above, I meant to reply to a different comment.)
I'm eating some right now, turns out. When I'm done, I'll edit this comment with a count of unpopped kernels. :-)
I'm using one called Salbree from Amazon. But looking at the pictures of the other ones, many brands of them look identical, like they're made in the same factory.
The process is surprisingly reproducible. I get very few unpopped kernels, usually around 5 of them.