There is a lot of data available on the web from various sources. To be most valuable to you, any estimates (guesses) of future cost of living (COL) changes must be based on your specifics. These specifics are location, what you currently spend money on, health status of the family members, and the best guess at future cost changes in these specific items.
I personally have broken down my costs into a couple of dozen categories and assigned a best guess future inflation rate to each. I did this in a spreadsheet to allow projections and to perform what-if scenarios. Of course expenses come and go and change with time and I factor those in as best I can.
All that glitters is not gold. -G. Chaucer, W. Shakespeare
All that is gold does not glitter. -J.R.R. Tolkien