When the plant has 4 or 5 sets of true leaves is the advice I'd seen. Alternating nodes would be pretty far along to be using plain water I think? That'd be about time to flower them, no?

I also think that there is a lot of room for experimenting
My little ones only have 2 sets of leaves and I'm using tiny amounts of 3 part. About 1/4 less than recommended on the bottles for 'seedlings'.

I dont know jack, but so far no visible harm and I'm watching like a hawk. I can view them all day long to watch for signs, so felt ok about it.