250w Horticultural CFL - Height above canopy?

[FloridaSunset]

Have an Apollo 250w Horticultural CFL grow light and need to know the optimum height above canopy it should be placed above the canopy during both the Grow and Flower cycles. The last grow I did {my first successful one} I tried to keep it as close as possible as you would with a normal CFL bulb but noticed some leaf burn so kept raising it until it was about a foot above the top of the plant. Even then, though the growth was alright I didn't get much in the way of bud swelling -nice bud size but not very fat if you know what I mean.

Any help would be appreciated. Thanks much in advance...