That same logic works of course regardless of what number you’re counting down by. If you can subtract without borrowing do so. If not add the 10s-complement (.e.g. 7 vs 3, 6 vs 4) and subtract 10.
Wow, I read this after I’d posted above saying -
That seems way more complicated than what I was doing. Which, now that I think about it, was just subtracting 7 each time.
Never mind. As I said, I really don’t get numbers.