two more points:
1) To see calendar drift we dont need to project to the future. The Tekufas Tishrei according to Tekufas Shmuel is on October 7/8. The actual equinox this year was on September 22 15 days earlier. In other words we say Vesein Tal umatar 15 days less than in the times of Shmuel. And every 400 years we will be saying it 3 days less
2) It just occured to me that Vesein tal umatar in chutz learetz isnt linked to any pronouncement by beis din. In other words. Im not sure it will change when Moshiach comes. The halacha is 60 days after the tekufah. For which we use Tekufas Shmuel. Will we change to using the astronomicla tekufa? or something else? IT sems strange to think in thousands of years those in chu”l will start Vesin Tal umatar based on the inaccurate tekufa . But why would it change? and is there a source that says so?