How best to self-time a 100?

Look up a guy called Matt London. Has been hand timing short sprints accurately for years.

Most likely underestimating the time. The biggest factor is the reaction time. Since you can anticipate when you'll start, you press start the moment you start moving. In a race there's going to be a couple tenths of a second lost to reaction, assuming you're a slow starting distance runner and not a world class sprinter. There's not much you can do about this by yourself, really. As far as finishing goes, you're right that you should stop it when your torso crosses the line, but again, you can gain or lose a bit of time there as well. Just take self-timed, hand-timed reps with a grain of salt, and know you're likely a bit slower than the times say.

There's fancy things like freelap that seem cool, but not worth it unless you're really sprint training. You're best off having a track-savvy friend click the stopwatch for you if you want it to be as accurate as possible.

Used to have a watch that would let you put in a countdown timer with a beep at the end, then go automatically into stopwatch count up mode. I'd set the timer for 20 seconds or so, then when I thought I was getting close to zero, I would get into my set position and wait for the beep. So it did take into effect my reaction time. At the finish I would wait for my torso to cross plus a fraction of a second more, just to be sure.

You have to add your reaction time to make it more like a race time. So add 0.2 seconds.

The most consistent way to hand time a sprinter in training, including yourself is to start the watch as the first step hits the track. This is easy to do, but then you need to add a full half second to estimate a comparable competition time.