If you do it when you're 75% of the way to FG, that would be the best time. So, if you started at 1.060, and expect a FG of 1.015, to do it at 1.026 or less would be fine.
If you're not checking the SG very often (and I don't!), waiting until signs of active fermentation slow a bit may get you very close to that, and would be a good time for the diacetyl rest.
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