I have a nearly similar light for my clones and i keep it about 4-5 inches from the top - when you're topping new growth and kinda new I'd recommend waiting until the 4-5th node before topping.
what are your temps? low temps are brutal at slowing down growth even if they look healthy.
if you're gonna use a pre-mix it's always safer to use one accustomed to cannabis specifically. otherwise just get whatever store has Fox Farms and call it a day. pro mix and coco are great - when you know what it needs.
what do you mean by liquid fertilizer? there's everything from snake oil to concentrated gold, the brand and product matters unfortunately.