DC Motor Car Steering
Basically you can't. PWM value required to turn to a specific degree will depend on motors, on voltage, on hardware, in the current velocity of the car and probably on the surface under the wheel. Also keep in mind that real cars don't turn like that (at a fixed angle). Instead, driver constantly adjusts the position of the steering wheel, while looking at the road (i.e.a feedback system). Same is for RC cars, operator smoothly deflects the joystick looking at the vehicle.