When doing retirement planning it is important to know what your annual expenses are.  I'd like to compute my expenses for 2017 but in a way that isn't tedious.

I'm thinking of doing this:

 1. Get balance of all accounts on Jan 1, 2017 -> B2017
 2. Get balance of all accounts on Jan 1, 2018 -> B2018
 3. Get return on investments (my bank provides this) -> RETURN
 4. Get income from 2017 tax return -> INCOME
 5. Get income tax (and SS etc.) paid from 2017 tax return -> TAX

Compute expenses as:

    INCOME - TAX - (B2018 - B2017 - RETURN)

Would that be a reasonably accurate way to compute my expenses for 2017?