MP,
Thanks for that detailed explanation.
I knew you had to consider what you detailed  and that multiple pulleys/ropes share the load of pack. I didn't know how to figure the working load or margin of breaking strength, so thanks for that.

However I figured becasue that Cooke Custom line is rated a 450# which is far more than most lines people tend to use for their bear rig, I figure with a 3 pulley rig I'd be hard pressed to have a food pack so heavy I'd be in trouble? Multiple packs maybe.

When I bought those sailing blocks/becket, I mentioned to guy what they were for and the rope I was going to use. He showed me some sailing rope of similar size that was rated close to a 1000#.
I didn't ask how much, but with that line and some good blocks you could hang your pack and your hammock/canoe with room to spare. ;) ;D ;D