My spreadsheet calculates nominal dollars in columns demarcated 2015-2050.
Each column has income, money in tax-advantaged accounts (with limits adjusted to inflation - i.e. 401ks increase by $500/year according to IRA rules), tax paid, and expenses. Income is adjusted for hypothetical raises, and expenses are adjusted for inflation depending on what kind of expense they are (for instance, rent would be adjusted for inflation, but mortgage payments would be listed in actual amount for the duration of the loan.)
Investments are increased by 7%.
The only thing it doesn't account for is potential changes in taxation over time, because I don't know that.
I do like it this way instead of simplistically making ROI 5%, because that assumes inflation acts identically on income, all expenses, and investment accounts. HOWEVER, I just made a separate spreadsheet using the simplistic 5% but no inflation method, to compare, and it's only off by like 1.5% total investment value. That really doesn't make a significant difference for the purposes of these spreadsheets. (Especially since if I'm assume investment returns are 7%, I'm going to get a TON more variation in actual results due to investment returns, not due to slightly-off inflation adjusted expenses.)
TLDR: If you want to be pedantic, you should account for inflation in nominal terms. But if you just want functional guidelines? Keep doing what you're doing, your predictions are just as useful for FIRE planning.