For a 3.3 V CMOS input, 
    - V_IH > 2 V
    - V_IL < 0.8 V
Assuming the ADC scale is linear in voltage and is zero at 0V, 
    - 2V    => 0x2FF (given) = 767
    - 0.8 V => 0.8 / 2 * 767 = 307 = 0x133