Use one unit (kg), fixed timing, and a short context note for each entry (diet, medication, activity).
A practical weekly method is: fixed once per week, fixed context (before meals/same time window), unified unit (kg), and one-line context in the same record. Context only needs factors that materially affect weight: diet transition, major activity changes, medication, post-op recovery, or stress events (boarding/moving).
To judge whether weight is abnormal, trend is more important than one-time fluctuation. For example, if weight drops for two to three consecutive records and appetite or energy also drops, clinical evaluation should be accelerated. If appetite and energy are stable, small weight shifts may only be timing or hydration noise.
You can also compress monthly weight, diet, medication, and key events into a one-page monthly summary to reduce review cost.