Most of the time when cutting for myself, I eyeball 16". I find where I tend to get messed up is when switching from a small diameter log to a big one (or vice versa): I tend to cut longer on the larger diameter log. So I'll sometimes use the bar as a gauge to recalibrate my eyeballs on the first cut after a switch to a new log with a significantly different diameter.
I've also noticed that some days I do better than others. If I'm having a really bad day, I'll pick up a stick and hold it in my left hand along with the front handle of the saw. I grab the stick so that 16" sticks out past the bar and use that for a gauge for a bit.
I also own one of these "Woodcutters Helper" magnetic firewood gauges:
Watch a short demo video. This video is of the original Woodcutters Helper and is forillustration only. Never struggle with a piece of firewood too long to fit your stove. It attaches to any location on either side of the chain-saw bar using a strong Rare-Earth Neodymium magnet rated at 48.5...
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I use it occasionally. It's main use these days is when I bring it along to one of our community splitting bees where we are processing firewood for our firewood donation program. I'll sometimes use it to put small cut marks on a bunch of logs for others to cut up, or led it to someone who is having trouble judging the length. It's simple, fast (easier than twisting the saw to use the bar)
An those odd chunks: On my own property the gnarly ones that would be a pain to split get cut out and left in the woods to rot. When I have so much excess firewood, they are not even worth thinking about, let alone putting extra labor in to. If I notice I'm going to have an odd-length chunk early enough, I'll adjust the length of my cuts shorter or longer to split the difference. Otherwise, they just get left where they fall or kicked off to the side of the trail. If I know I have a bonfire/campfire coming up and have some room in the trailer, they might get hauled back home.