I'm similar to Nam, I start at about 27" in veg and then let them grow up towards the light keeping about 12" away in flower but if I notice any bleaching I'll raise it up a bit further.
My LED have 90 Degree lenses, lenses with a 60 degree beam angle, would go higher due them being a more concentrated beam.
I'm also thinking 5W LEDs may go higher too but I don't have one so can't say for sure, hopefully a 5W LED user will chime in here.