I'm in the same game, but your number needs to be adjusted. Lead melting point is only 621 °F (327 °C) and is a bit higher if mixed with zinc and antimony. At 510 deg C before I learned better, I have watched lead vapours coming out of the pot.
I think @cuslog's numbers are correct. Just like ice melts to become liquid and then boils to become vapour, lead melts at one temp and then vaporizes at a much higher temp.
You still need to worry though. Some lead does end up vapourizing at the same time as it melts because air can hold lead vapour just like it holds water vapour in the form of humidity. No idea what the humidity equivalent is for lead though.
What I do know is that I have no interest in sniffing liquid lead!