The key things to record are
- The account balances of your key bank accounts and investment assets
Things you should do your best to record
- All your approximate earnings and income, both personal and from investments
- All interbank transfers and asset purchases
Things you don't need to record
- Every single expense. Just the one off large items might be worthwhile
Things that are nice to record
- Net income is okay (i.e. gross-expenses)
Account balances allow you to determine your networth at points in time
Recording transfers and asset purchases is needed to avoid spikes in your networth caused by reading account balances at times before and after the transfer
Your earnings are needed to determine your monthly/yearly income for forward planning
With all that information, your overall expenses can be determined. Any specifically measured expenses are just for your reporting granularity
By knowing your networth, income and expenses, we can forecast many things like
- Return on your investments (IRR calculation)
- How close you are to retirement