How much support needed for hanging shelf?

We're thinking of building a hanging shelf in greenhouse. How to figure out how much support is needed?

Shelf would be \~8ft long suspended by air craft cable from 2"x3" \_horizontally\_ mounted spruce ceiling rafters. The shelf would be holding 5 or 6 buckets with plants (so up to \~250 lbs if they were all filled with water from an over enthusiastic auto sprinkler system ;-).

Basically the question is how many support cables will be needed? Can we get away with only the far ends, or do I need to go ends and middle? or every single roof beam?

I've seen [The Sagulator](https://woodbin.com/calcs/sagulator/) courtesty of other posts in this subreddit, and plugging the numbers above seem to indicate we'd be fine with end-only support. However I have this niggle that's the wrong calculation approach for this scenario with suspended vs supporting and so on.

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