Have you tried this ?
float voltage = sensorValue * (5.00 / 1023.00);
The reference voltage is 5V.
The voltage you want to read 3.3V, is in the reference range, below the 5V, therefore no need to adapt the input voltage.
In the circuit of the website they want to measure a 9V voltage.
But, the reference voltage for for A/D converter is 5V.
Therefore, they divide the battery voltage by two down to 4.5V with the voltage divider, to get a voltage in the reference range below 5V.
And after reading the use:
float voltage = sensorValue * (5.00 / 1023.00) * 2;
They multiply again the voltage by two.