Unfortunately there is no set graph/chart for LEDs as they vary so much from maker to maker (size of bulbs used, lens material, reflective material, etc.) so what you often see is to contact the manufacturer for guidance.

As a very general guideline - 300W can easily burn plants if too close so a common test recommendation is 30" above. Then monitor the plants closely for any signs of stretching and move down to 25-28" if necessary. Any signs of leaf sunburn raise them up to 34-36".

But you also have to keep in mind the footprint of the fixture (rotate trays as needed), type plant and its different light needs, humidity and air circulation, etc. as all can effect response to the lights.

You might want to do some reading of the past discussions of this issue over on the Growing Under Lights forum here for more info.

Dave

Wow! 300 watts is a lot for an LED bulb. It must be a panel. If it's like lower wattage LEDs, the base will get a lot hotter than the lights themselves. I'd just try it out (starting off a distance away, and gradually getting closer to see how close you can get without any issues).