Couldn't find Sena's book but I found this on the net.
"In Hebrew, each of the 22 letters of the alphabet has a numerical value. A code called gematria, developed in Jewish apocalyptic and mystical circles, draws connections between two otherwise unrelated words based on the total numerical value of the letters in each word. Nero's name or title in Hebrew looks like this: (NRON QSR), meaning Nero Caesar. Taking the value of the Hebrew letters, N=50, R=200, O=6, N=50, Q=100, S=60, R=200, one comes up with the sum of 666. To make this identification of the beast with Nero even more convincing, it should be noted that some Greek manuscripts of Revelation give the number 616 rather than 666 - probably due to an alternative spelling of Nero as NRO rather than NRON (666 minus the final N [=50] gives 616)."