Generally the soil has enough nutrients for vegging a girl for a month or so.Should I have been feeding from the beginning even though I was growing in ocean forest from the first day
But going into flower its generally considered a good idea to start feeding.
But there isn't a hard and fast rule for when your soil will run out of nutrients.
Most deficiencies appear in flower so you can't go wrong feeding from flip 12/12.
I don't think it would hurt to give her a little calmag every couple feedings.
Bill