Correction below on setting metronome; 30 beats a minute, NO 120.
For using your bulb setting and cable or bulb release, the most practical way to get a fairly accurate handle is to use an audio/visual metronome.
I have a small credit card size Sony metronome I bought many years ago, for guitar and it's ability to allow you to get your desired split second timing correct is child's play.
For 1/2 second, set it to 30 Thirty beats per minute, start it up, with the camera and release ready and when you're sure in your head of the intervals, fire the shutter and release in time with the progression marks.
This tool is no locked into a set of beats and you can configure with uneven steps as/if needed.
I use mine with my Pinholes and regular camera's, for longer exposures and the audio can let you take your attention off a watch, clock or cell phone, and focus on your image/view of the subject you're photographing.
I suggested this as a method of adjusting slow shutter speed settings, to the one repair person on YouTube, and I plan on giving it a go, once I find a thing that goes "bbuuuuuuzzzzzzzzzz" in my slow speed shutter, under repair now.
People are very good with keeping time with beats, and there's no reason this should no work for you.
A good cell phone app should be just fine too.
Eli