Start with where you want to live and what you want to do there. Spend some time thinking about your average day. Do you plan to travel, socialize, dine out? Or do you envision spending time at home, trying out new recipes, or taking up a new hobby? From there, estimate if your daily living expenses will go down, stay the same, or go up, compared with your current living expenses.
A retirement spending plan will help you get a handle on how your projected expenses align with your projected income. Remember to factor in health care expenses, which tend to rise as you age.