Everything is moving - income tends to increase with inflation, as do interest rates. So you have two options

Pretend everything stays in todays $, or...

Pretend there is a predictable inflation rate, income increases with inflation, use current interest rates, and each year adjust all your assumptions to the inflation that happened that year.

I always did the first.

I've always done the latter, because it's the only way your calculation makes sense along the way. Otherwise in 2015 you're making projections based on 2008 dollars from when you made the spreadsheet, and you have to calculate your current net worth in 2008 dollars. How confusing is that?

The other counter argument to the OP's question, discussed here often, is that your expenses do not have to inflate over time. Most of the retirees here retired by some combination of growing their stash and reducing their expenses. For these people, inflation was negative.

But even if it's not, it's not hard to calculate. Make a spreadsheet with a column for expenses each year, and inflate that number by your chosen inflation rate. Make another column for your actual net worth in that year, either observed or calculated using actual (aka "nominal" aka "what is reported on your investment statements") investment returns in current dollars. When column B is 25x column A, you're there. Easy.

From there you can complicate things if you want. For example, if you're keeping a mortgage in retirement then it is unlikely to increase with inflation (other than property taxes) so you can subtract off that fixed expense and only inflate the rest of your living expenses.

I've never understood how anyone can make sense of a future calculation using "real" returns with inflation subtracted out. What a headache.