LCDs measuring volts arduino
The project that you show as your reference : link in your question,  shows this picture :  

  

Change R1 to 200 K ohms.   The Arduino will receive 4.76 volts when the input is 100 volts.   

You will also need to change your program line for float R1 = 100000.0;

CAUTION :  Beware of high voltage.  Also, the Arduino is not isolated from the 100 volts.