which is the proper distance from lamp to the top ...

As close as you can get without causing problems and covering the entire plant. I know, that's vague, but it depends on too many factors. Start with what the manufacturer suggests, then allow what you observe from the plant to dictate adjustments. Read up on what too much light causes. I'm sure you can recognize stretching from too little light. In bloom, it will cause buds a bit lower to seem more ripe and be coated in more trichomes. The top is stressed and throwing new pistils while the trichomes are being evaporated at a faster rate that causes the difference seen. Your top colas should be the most developed and most covered in trichomes. If the opposite is true, that's almost certianly heat or light and both are caused by the same source in most cases. photons are neither lost nor gained (laws of conservation of engergy and mass apply to photons), but you will lose some to inefficient reflection off your walls. Get the light footprint to more than cover the canopy and provide some bounce to mid/lower regions on sides, but canopy is the bigger concern -- probably best bet. As long as you aren't wasting photon on the walls, you are not too far away. All those photons are stil there, just spread out more as they travel further. If you see light burn and other issues, you know you have to draw it back and be a bit wasteful. Grow a bigger plant next time to fit the light better (more likely it's the opposite) You want light hitting more than just a single layer of fat leaves on top. If that's all that happens, you get 6-8" colas that are rock solid colas, but you are passing up 16-24" colas with better light penetration and excellent air flow.