I would not trust an AdSense RPM calculator more than my last thirty days in the dashboard. Calculators help you play with variables. They do not replace your country mix and niche.
Use your real RPM first
Export recent earnings and pageviews. Divide. That is your baseline.
Model three scenarios
Low, typical, high RPM. Plan expenses against low.
Watch the inputs
Pageviews without traffic quality lie. RPM without seasonality lies.
Calculators are spreadsheets with a UI. Feed them honest numbers or ignore the output.