Weight of car measured under both REAR tires when car is perfectly horizontal and scales are positioned on a smooth, level floor. Measurement should include driver, full tank of fuel, on-board fire extinguisher, helmet, etc.

(Weight_REAR = Weight_{REAR  LEFT} + Weight_{REAR  RIGHT})

IMPORTANT

For vehicles that are meant to carry more than just a driver, his/her helmet and the other necessities that are usually found in race cars, always load it up with the maximum weight it will be able to carry! (look up its GVWR)
Spread the load the way you would when loading the vehicle to its maximum operating weight.

At maximum weight, a vehicle still has to be able to decelerate as fast as its tires allow, locking up the wheels or running the ABS (emergency stop, for example), on clean/dry tarmac/concrete, without creating a situation for the driver (any driver!) in which it is hard to apply the required pressure to the brake pedal.