In my net worth estimation spreadsheet, I used 6% as a default market return and recommended that you keep it conservative. If the market happens to be better that that, that’s good news. If you estimate it too high, then you might not save enough.
Another way to model this is to use historical market returns year-by-year.
The site FIRECalc does this for understanding post-retirement spending vs. expected market growth. It takes your portfolio size and starting expenses as input and then draws a line on a graph for each possible retirement year using real market data. You get an idea of what would have happened in all possible historical scenarios and what percent of the time you would not have had enough in your portfolio.
If you want to do a more sophisticated analysis, there are advanced versions on the site. For example, it assumes constant (inflation-adjusted) expenses, but if you want to do something more sophisticated than that, it offers a few spending models. You can also adjust your portfolio stocks v. bonds balance.
Doing this kind of analysis is the kind of thing that’s fairly easy to do in a program, but not as easy to do in a spreadsheet. It’s not impossible, but you would need a couple of columns for each scenario. In my version, to show 30 different saving scenarios, I need 60 columns, plus a few columns for the actual historical data.
I’ll work on updating the spreadsheet to show a few scenarios to give you an idea.