All I would say is to make sure that your plant has been been in true vegetative (time since the first true leaves formed) for at least 4 weeks, and has at least 4-5 nodes. Plants can waiver on sex around the 3-4 weeks mark and any stress before/around this time is likely to cause problems.

I would recommend experimenting if you have a few plants of the same strain; try one at 4 weeks, one at 6 or so, one at 8, etc. This way, you will know what works best for your particular setup and strain.