Here in the inside passage of Alaska..420 - 475 works the best for spot shrimp. FYI..prawns don't live in the ocean. If you use weighted line you only need to use one small anchor above the pot..that way the anchor stops the pot from being dragged by the float. Cheers.
The anchor works better if it is placed on last..so the bouy can't move the traps..they way this anchor is done on this video the traps will be constantly moving and the shrimp won't like that and start to leaving .