Hey.
I added the 3rd position yesterday afternoon. Keep in mind the hpa roots were not getting 360^ coverage whereas the F & D were/are. So it is not fair to judge one against the other, BUT, I expect that the hpa will begin to take off. This morning they were quite perky, better than with 2 @ 2 heads!
The reason I went with 3 heads is I simply added a 3rd head 'T'ing off the other 2 lines that were already there. I think this method balances the pressure to all 3. Thinking further on this, the 3rd head (the one in the middle of the long side) could be a dual head to provide max coverage. Decisions decisions. I may play with this later today.
If I went with one incoming line (instead of two - one to each short side) then I could place 4 heads, but might lose pressure to the 4th (farthest) head. Also it has to do with number of plants and their location on the lid/table. I plan to replace this R & D tote now that I have a better understanding of max adult plants in this size (~33G) is probably 3. This one I set up for 4 plants, but I think 3 is max to get good nute circulation. Also the holes in the side walls were experiments for mist head placement: higher is better, but not too high.I have taped over them as they were leaking light.
Assuming my hpa begins to show positive signs that I am finally on the right track, then I will consider replacing this tote and giving it one more shot with some clones. After all I have invested several years of experimenting, and this seems like the last thing I can do without resorting to adding an accumulator set up.