I always rack wines when the sg is around 1.020. The reason is that there still should be enough co2 being produced by fermentation so the wine doesn't oxidize.
The problem is though, you might actually inhibit fermentation by doing that with mead. I think that with the recipe you're using, I'd rack at 1.010 or less and add a pinch of nutrient so the fermentation doesn't stall. Someone else on the forum (I can't remember the name right now, but hopefully they'll chime it!) racks when the mead is done fermenting, I believe.
If you're following a recipe, I'd try to follow that. If you're doing your own recipe and just looking for technique, I'd probably wait until the mead was about done just to make sure it's not going to stop.