I am planning twelve nights in Paris, Lyon and Nice, then a two-night stop in Milan before flying home, but the cheapest flight lands in Rome at 09:00. I feel worried that the first border check will question a French application, so I want my hotel nights, internal trains, return ticket and explanation for the Italy arrival to be consistent before submitting anything.
Choose the responsible application country from the genuine main destination or main purpose under the current Schengen guidance, not from the cheapest flight; then make the itinerary show the real nights and transport. If Italy is only the first entry point, keep the Rome arrival, onward train, French accommodation, funds and return booking together and be ready to explain them. Checklist: application form, passport, insurance or other documents required locally, accommodation and transport evidence, with dates that agree everywhere. A changed itinerary or equal-time destinations may require a different analysis, and border officers can assess the actual trip: https://home-affairs.ec.europa.eu/policies/schengen-borders-and-visa_en.