I have a written budget for our estimated retirement expenses. To put it together, I started with our current expenditures, and then made some assumptions. There will be no more college savings, but we'll want money to travel to see the kids, so I reallocated that. There will be no more mortgage, but we will need to provide medical insurance, so I reallocated that. Our grocery bill will go down with the kids gone, but we'll be eating out more, so I made that adjustment.

I think if you start with your current expenditures, you can make similar assumptions based on what you want to do in retirement, and reallocate your money accordingly. I do like to make a note of my assumptions right in the spreadsheet so that I can make adjustments later as my assumptions change or I get more data.

This might be a reasonable place to start.