I think most honest commentaries have long realized that apocalyptic writings such as Revelation contained purposely vague statements to protect them from their persecutors while driving the messege home to people who sympathized with them.
The 666 reference is just such a notion.
Gematria was popular in Rome and among Hebrew peoples because of a quirk in their alphabet: they used letters for numbers!
Mystics played around with this with too much time on their hands for anything but superstitious nonsense.
The assumption became this; the bible had significance NUMERICALLY as well as LITERALLY.
It only left them with the task of finding a signficance numerically to complete their "understanding".
The Emperor NERO was a persecutor of Christians to the extent he blamed them for starting the fire that consumed Rome at the time.
If you write the name and title of this emperor in the manner of the time (IN GREEK) it would be:
Now it is only necessary to convert this into Hebrew letters (numbers) to see that the total "value" is 666.
Some early manuscripts of Revelation give the number a 616 and this is what you'd expect if the name were written as: