50w of cobs is more powerful than most LED companies 300 claimed watts. So you basically have a mars hydro 1200 in there now lol for 200 watts at the wall. I have some specs on these 2 lights and the comparison is in extreme favor of the cob as far as light, spectrum and intensity goes.
Im not to familiar with grownorthern but I do know you can only get so much out of a watt, spectrum seems good but no single led is really outperforming these cobs by any significant amount.
Once the wattage part is thrown out the window a good starting point is probably 30". Watch the plants to see if they stretch or burn. Every strain is different, reflective or wall material, plant configuration all plays a huge roll in height. The other thing is you don't want to be over cautious and lose all the benefits of the light by having it way to high either. What I do is start around 30 and slowely lower it till I see the plants change. It takes time and effort to get dialed in but it's worth it. Google inverse square law to understand light intensity at different heights.