I didn't mean to get off subject there. I start LST training when they're pretty young and the stems are nice and soft and I'm not a real big of a fan of pruning or topping. I'm running a 450 CFM exhaust fan on my 1000 W high-pressure sodium with a hood. You can actually touch the glass on my hood keeps it so cool; hopefully this will keep up during summer. With as much light as you are running, those things should veg pretty fast. My personal opinion; I usually don't clones or would not top until they were at least 18 inches tall, depending on how much room you have to grow in.