I have a 2018 single motor
I have a 2018 single motor long range Model 3 with 19" wheels. I took delivery Sep 14th 2018. Reported range at full charge is almost the same as yours, within a couple of miles. When delivered, a full charge was 310. There was a short period when a software update was supposed to have increased full charge range to 325, but that didn't last very long. I've always charged to 90% unless preparing for a long trip.
In addition to the small loss of range, I'm impressed with the energy efficiency of this car. A recent trip (using TeslaFi to collect data) of 200 miles, running 70MPH, charging at home, cost me 2.88 cents per mile. Amazingly cheap transportation!