Test your method against two known wehicles: Model S and Model X. Their dimensions, weight and efficiency are well known and published.Besides the drag area, if we consider the drag coefficient numbers as well (0.21 and 0.24), it looks like the Model 3 will be 15% more efficient excluding any additional efficiency because of less weight. Ignoring weight, the RWD Model 3 55 could achieve 210/60*55*1.15= 221 mi EPA but the 55 kWh battery weighs less than the 60 and the rest of the car should weigh less too. However, weight doesn't affect range that much. So my best guess for Model 3 55 is 227 mi EPA.
You ignored one *very* important factor: rim/wheels size. Change 19" with 21" rims on the same car and you just lost 10% efficiency.