I didn't even factor in the top part of the fixture where they will hang from & rotate around PVC for 100% even light spread & distribution all around the buds even from the side angle the lights will be hanging from
Yeah, an HLG-320 could be enough. It's for sure not really 33sft of canopy area. That what I've calculated was the outer surface area of a cylinder with diameter of 21" and a height of 72“. The inner area is for sure a lot smaller.
But sft or m² are the only way to calculate how much light you need.. also for vertical setups.
You're right, a light mover will optimize light usage so that you can get away with less watts per sft but it's still the way to calculate such things. You need to figure out the true diameter of the inner cylinder when covered with plants.
If the diameter is lets say 15" the outer surface area would be ~23,5sft and because of the light mover you could get away with 20-25w/sft maybe. In this case you could use an HLG-480 and up to 550w
It really depends on how much space the plants need inside the bird cages and how much surface area they cover when the stretch is done. Maybe it's better to have the bars hanging in a way that allows to also move the bars back and forth; closer to the canopy or farer away.
Weed grows like crazy and such a cylider covered with 20" high plants can easily burst such a small bird cage. They grow in the direction the light comes from and 20" is pretty small for an MJ plant. You're anyway already forced to use low stretching indikas with such a setup.
This plant below is grow from seed, strictly 12/12h, coco DTW and handwatering up to 5 times the day and she has still 4 weeks to go before she is finished. Only 21" high because of the small 16oz cup but that's already a bonsai lady. So that's the minimum height you should calculated with.
Now imagine such a plant but lots of them growing in all 4 directions ... the outer diameter easily reach 45-50" or so. In this case such movable bars like describted above are neccessary otherwise there is no way to rotate the fixture as soon as the plants meet the bars.
I hope it's understandable so far what I mean cuz english isn't my 1st language/mother tongue. I really believe you need more space between the inner "root cylinder" and the rotating bars.