It's pretty obvious that the showroom adjustment is calculated based upon more than just mileage; likely the age of the vehicle, mileage and any features that are missing that would be considered standard on a new model. The exact calculation probably varies by trim level, but for the Model X P100D I purchased and the others that I considered at the time, you could get within a few $100 of the showroom adjustment if you deducted 1% of the base price per month since manufacture, $1 per mile and the price of any options missing at the time of manufacture that would be considered standard equipment at time of purchase. That may not be the actual calculation, but it matched the dozen or so P100Ds I was considering when I purchased.
My new inventory model was a demo car with 7700 miles, but I've seen them offered with 15,000 miles or more. A car that has been previously titled is pre-owned, one that has not is new. With traditional dealerships, it's possible for a car to be placed into service (as far as the manufacturer is concerned) by the dealership and then still sold as new. In those cases, the manufacturer's warranty is usually calculated from the "in-service" date and mileage. Tesla calculates the warranty (both age and mileage) from delivery, so even demo cars have full warranty. I suspect that's a result of not having independent dealerships. There are some EV rebates that consider the vehicle mileage and a demo car may not qualify even if technically new. For the federal tax credit though, all that matters is that it hasn't been previously titled.
