I am with Ampster. You can do the calculation directly based on battery capacity without considering rated miles. Much of the "controversy" about the Wh/mi corresponding to the rated miles displayed on the dash relates to how much of the nominal battery capacity (60/70/85 kWh) is available for use before (a) the car shows 0 mi range / 0% battery or (b) the car stops to protect the battery from permanent damage. Based on what I have read in these forums, Tesla may have reduced or removed most of the buffer associated with (a) that was originally available so that (a) and (b) are almost the same with the current software.
A good rule of thumb appears to be that only 95% of the nominal battery capacity is available for use - 81 kWh for the 85 kWh battery and similar for other capacities. For the 85 kWh battery at 400 Wh/mi that gives a range of 81000/400 = 202.5 miles. Of course you will want to build in a 10-20% buffer to account for the unexpected, from strong head winds to detours to problems with the planned charging location (ie don't plan to use 100% of your estimated range to get to your next charging point.) If your trip involves significant elevation changes, you will want to factor that in as well.
There are two resources that can help you beyond the basic range calculation. The first is in the car itself, assuming that you have the navigation option. In the energy app there should be a second tab that says trip. This only works once you have a trip to a destination set up. It will show your expected % charge remaining when you reach your destination. This estimate includes elevation changes. Unfortunately, the initial estimate appears to be based on the nominal energy usage for your drive configuration, but as you start driving it corrects for your actual energy usage. If the estimated %charge at the destination drops below your comfort level then you have two options. One is to add a charging stop sooner, say if you were planning to skip a supercharger because you expected to have enough range to reach the next one. The other is to slow down slightly, since even a drop of 5-10 mph at highway speeds dramatically increases range. This is a great way of getting early warning that something that you overlooked is causing you to use more energy than planned, allowing you to take corrective action before you have a crisis.
The second resource is EVTripPlanner (
https://evtripplanner.com/index.php). You can configure this for your car's configuration, including load. The estimates take into account elevation changes (but not wind or rain). Also it does not include your roof box. The best way to model the roof box is to select a planned speed that is higher than your actual planned speed. Plugging your test results into the planner should allow you to determine a good adjustment factor. The planner reports estimated Wh/mi for a projected route, so just adjust to get the observed usage for your test route. This tool is very helpful if your route has significant elevation changes.
Sounds like you are doing all the right planning to have a successful trip, even if superchargers are a bit sparse along your route. Good luck and have a great trip.
