It would drive me right up the wall to not know precisely how and where I spend my money, but I accept that some people (maybe most people) are not like me. If I were unwilling to track expenses and wanted a simple SWAG for planning purposes, I would do this.
A Look at my end of year pay-stub for 2017 and note the net income for the year. i.e. - the total of all the paychecks deposited into my checking account for the year.
B. Did I receive any funds into the checking account other than paychecks during 2017?
C. Did I send any funds from my checking account to savings in 2017?
D. Look at my checking account balance on 12/31/16 (D1) and again on 12/31/17 (D2)
From the above: Spending = A+B-C-(D2-D1)
Caveats:
This assumes a single checking account through which all income and spending flow. I use the net paycheck amount, which excludes the amount paid into 401k/403b etc., the amount paid for healthcare premium, the amount paid toward my pension, social security and medicare withholding. Most importantly, this also does NOT include state or federal income tax which are taken out of my pay before I get a paycheck. If you do this to plan your retirement income needs, you MUST "gross-up" for income taxes, because they will not go away after retirement.
Also note that this provides no granularity, so you won't be able to adjust for spending that will go down after retirement (commuting, professional dues, dry cleaners, etc) and that which will go up (health care payment, travel, etc.)
** If you really wanted to get fancy, you could run the above equation for 2 or 3 separate years and use the average.
EDIT -- reading back through, I see that retirement for the OP was in mid 2017. So this exercise could be done for 2016 and prior for her.
