If you are going to keep a dose log at all, it is worth deciding up front which fields you record. A log with missing columns is only slightly better than no log, because you cannot compare anything across time.
The fields I would not skip:
- Date and time of the injection
- Compound and vial identifier (batch or the date you opened it)
- Concentration in mg/mL for that specific vial
- Dose in mg, and the units you actually drew
- Injection site
- Weight, if you are tracking it, taken the same way each week
- Side effects, in plain words, with severity
The two that people leave out and later regret are concentration and vial identifier. Without them, a dose in units is meaningless six months later, and you cannot tell whether a bad week belonged to a particular vial.
Site rotation is worth logging simply because you will not remember. A four-point rotation written down beats a mental note every time.
A spreadsheet is fine. So is paper. The only real requirement is that the same fields get filled in every time, which is why a purpose-built dose log and calculator tends to survive longer than a spreadsheet that you redesign every month. Whatever you pick, back it up somewhere that is not just one phone.