rockwool and foxfarm drip system questions.

I have some purp clones that i just transplanted into 4x4x4 rockwools and was wondering how long until some roots will show?

i feed them fox farm grow big and big bloom at a 1/4 strength just until i see some roots. is that ok?

Should they be flat against my grow tray or should i elevate to get air between the tray and rockwool?
 
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