Well it's hard to say, Based on the PPFD charts that budmaster did, it looks like 60cm would be ideal for 12/12, Generally for flowering I try to keep my plants above 600 PPFD and below 1100 PPFD.
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Now I don't know what meter was used for these tests or method of reading these. I did actually do readings with my Apogee MQ200 which is not the most accurate, and here are my readings at 12" 24" and 36". Now the Apogee MQ200 is not that accurate, so depending on what meter Budmaster used, they probably have more accurate readings as I've chatted with Paul many times over at Budmaster and typically he tries to use the best available so I would imagine he probably used a Licor or spectrometer. (but I'm not sure either, just a guess).
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So by comparing my charts with a inaccurate meter, and the supplied ones, I would say that 24" probably is a safe height. You could always go lower and see if you get bleaching. For veg you definitely want to raise it to closer to 3' as veg with a longer day doesn't need as high of PPFD because the light need is based on DLI.