To my understanding, a lot of the idea of using half strength nutes is because auto's in general can be more sensitive to over feeding and nute burn than photos. Half gives a nice, mild starting point and then you can gently increase if it seems like the plant needs and/or can take more. Varies from strain to strain much like some can handle Ocean Forest soil well while others burn up in it.
What I've been doing that seems to be working is I wait until the second set of leaves are starting to grow nice. Real leaves, I don't include the rounded ones that are the first to show. Once I see the second jagged set up to about 1/4 inch or so I start with a light quarter strength nutes, run that for a week and then if the plant is responding well go to half strength. The line of nutes I use (Earth Juice...also an organic line) has a foot note under their feeding chart saying to only use half strength for hand/soil feeding, so I've not gone over that yet.