Arduino - Voltmeter

The following code illustrates both the reading of a voltage with the Arduino and then adjusting it using the code from the Secret Arduino Voltmeter article by Scott Daniels:

Do some initialization stuff (obviously this is not Arduino code as I am using my interface library):

    import time
    from Py2Ard import interfaceClass
    my_board = interfaceClass('/dev/ttyACM0')

Get the reference voltage and use it to calculate an adjustment from 5 volts:

    if my_board.referenceVoltage() != 0:
        print "Error"
    else:
        print
        print "--------------------------------------"
        refVoltage = float(my_board.returnValue) / 1000
        print "Reference................. {0:.3f} volts".format(refVoltage)
       
        adjustToRef = 1 + (refVoltage - 5) / refVoltage
        print "Adjust to Reference....... {0:.3f}".format(adjustToRef)
        print "--------------------------------------"
       
Get the voltage we are trying to measure and adjust it using the above factor.  Note that the '* .0049' converts the analog return from reading the voltage to a five volt scale.  The '* 2' then doubles that value to compensate for the voltage divider factor of 50%:

    if my_board.pinModeInput(5) != 0:
        print "Error"
    if my_board.analogRead(5) != 0:
        print "Error"
    else:
        mainVoltage = float(my_board.returnValue) *.0049 * 2
        print "Input - As Read........... {0:.3f} volts".format(mainVoltage)
       
        mainVoltage = mainVoltage * adjustToRef
        print "Input - Adjusted.......... {0:.3f} volts".format(mainVoltage)
       
The actual voltage at this point in time is 7.82 as read from our meter so calculate an error percentage:
    
        actualVoltage = 7.82
        error = (actualVoltage - mainVoltage) / actualVoltage * 100
        print "Error from Actual......... {0:.1f}%".format(error)
        print "--------------------------------------"
        print
       
The output from the above script is shown below.   Note that I have also adjusted the Arduino referenceVoltage script provided by Scott Daniels as he suggests in his article (adjusting for the inaccuracy of the 1.1v reference by actually measuring the 5v Arduino output and adjusting for the difference).

    --------------------------------------
    Reference................. 4.910 volts 
    Adjust to Reference....... 0.982
    --------------------------------------
    Input - As Read........... 8.026 volts 
    Input - Adjusted.......... 7.879 volts
    Error from Actual......... -0.8% 
    --------------------------------------

Caveat comes here...it feels like I am double adjusting things to arrive at the answer that I want rather than the right answer...  I may post this on Scott's page and ask him if my not electronically oriented brain has miss interpreted things badly!