A lead thermonmeter has been a good investment for me. I was surprised to find that my melt was much hotter than I thought it was (1,000 F!!!). I find I am getting the best casting of RB's at around an indicated 750 F, which is hotter than some may recommend. That said, I have never calibrated this thermometer. But by way of results, that's were everything seems to be doing well.
Supposedly, lead begins to out-gas fumes at around 900 F. There is always some lead right on the hottest part of the heat source that I would expect stays well over 900 F, so I ventilate fervently.