It is above 750F where the very important austenite starts to form. Though far from exact (given the steel and its specific impurities, it's the best I've got. The correlation I use is as follows:
752F 400C Red: visible in the dark
885F 474C Red: visible in the twilight
975F 525C Red: visible in the daylight
1077F 581C Red: visible in the sunlight
1292F 700C Dull red
1472F 800C Turning to cherry red
1652F 900C Cherry red <img src="/ubbthreads/images/graemlins/thumbup.gif" alt="" />
1832F 1000C Bright cherry red <img src="/ubbthreads/images/graemlins/thumbup.gif" alt="" />
2012F 1100C Orange red <img src="/ubbthreads/images/graemlins/thumbup.gif" alt="" />
2192F 1200C Orange yellow
2372F 1300C White
2552F 1400C Brilliant white
2732F 1500C Dazzling white
2912F 1600C Bluish white
Interpretation of these colors is yet another variable in the HT. I do not think this is in any way as accurate as a proper forge oven, but I'm cheap and ovens are expensive.