I went back out for a run this morning and took my metronome with me to see if I could gauge my typical SPM and then to follow a set pace for two laps of my 1.5 miles doorstep loop (the actual distance could be plus or minus 20%, I just don't know the exact distance).
It looks like I've been running at somewhere between 176-178SPM as I tried running at 174SPM but it just felt too slow, whereas I know from running at 180SPM yesterday (and this morning), 180SPM requires more effort than usual. So my best guess is I've been running at somewhere between 176-178SPM.
I didn't get to time the loop at 180SPM yesterday as I didn't press the start button on my watch properly, but I managed to time both laps this morning, which I ran as consecutive laps without breaking cadence. The time of the first lap was 12.58.xx while the total time was 25.57.19, and when you subtract the time of the first lap from the overall time, it means I ran the second lap in almost exactly the same time - almost certainly within half a second. An astonishing outcome over that time and distance. Precision pacing at its most precise (figuratively speaking, as it could have been more precise).
I'm sure pros can do that level of precision or better without a metronome, but I ain't a pro and ain't ever likely to become a pro, so using a metronome on training runs is a no-brainer for the immediate future.
Incidentally, 180SPM at my standard stride length felt fast and although I could probably sustain it over 10K during a race, I might need to spend some time training a 178SPM first, and only once I've consolidated running at that cadence will I then increase my cadence to 180SPM. I'll eventually practice running at higher cadences, but I'd like to become comfortable at 180SPM first to make sure I don't injure myself.