Measurements alone do not make a finished outfit look right. Style preferences and reference photos deserve the same organised treatment to save you from rework and awkward follow-ups.
Once your shop grows past one person, tasks stop living only in your head. They get passed around, and anything passed around by word of mouth eventually gets dropped.
The tailors who rarely mix up orders aren't necessarily the ones with the best memory — they're the ones who built a system that doesn't depend on memory at all.
Your notebook has probably served you well. But if you're finding yourself searching for lost pages more than you'd like, it may be worth trying a system built for scale.