Suspect Arval have a standard calc based on their own algorithm plus data from Tesla, and the date will become more accurate as the weeks pass.
12-16 weeks is not a crazy wait, realistically you could have the car in 6-10 weeks.
What you need is your RN (Tesla reservation number) and make sure that Tesla see this as “matchable” from the lease company, and that the lease company have told Tesla it can be matched to a car being loaded onto one of this quarter’s ships.
The next stage after RN is a VIN to be allocated I.e. the matching process needs to happen (a combination of random computer swizzle and some sort of Colonel Sanders secret recipe), then you have a physical vehicle associated to your order - this can change but from that point on you are on the right track.
Once you have your VIN then Tesla will know which ship it’s on, then you can spend 3 weeks tracking the ship, then it’s another 7-14 days from there, and that’s the period when you start getting text messages confirming all the delivery steps.
You can choose to get involved in chasing up all those things or just wait for the hallowed text message of joy if you like surprises