Sensor Tranfer Function
I couldn't find a proper datasheet for the part you listed, but you can figure out the transfer function from a code example on the wiki page here: http://www.seeedstudio.com/wiki/Grove_-_Rotary_Angle_Sensor

int getDegree()
{
    int sensor_value = analogRead(ROTARY_ANGLE_SENSOR);
    float voltage;
    voltage = (float)sensor_value*ADC_REF/1023;
    float degrees = (voltage*FULL_ANGLE)/GROVE_VCC;
    return degrees;
}


The code implements the inverse transfer function of the sensor followed by the ADC.  So it looks like the sensor's transfer function of angle to voltage is:

$$V_{out}(\theta) = V_{DD} * \frac{\theta}{300°}$$ 

If you are connecting this to a 10-bit ADC like they are, you might also need the ADC's transfer function.  Assuming \$V_{ref} = V_{DD}\$, that transfer function is:

$$\text{Code out} = 1023 \cdot \frac{V_{in}}{V_{DD}}$$