When to use PK and start to flower?

When it is big enough for you. Plants can be put into flowers after a few weeks from germination. However, iff you can accomodate the size, letting her grow a few more weeks can double the yield but you will have to train her. Look at the strain data, see how much this strain usually stretech during flowering. Fertilizing, I mix my nute during the first week of transition. Half grow nute, half PK flowering nute. Second week, I use a quater grow nute ... and so on. Cheers