With wefts of human hair, going by weight is a bad idea. The longer the hair, the narrower the weft will be, and the fewer dreads can be made from a package. You want about 6 feet (wide) of weft for a full-head install.
With kanekalon, I used 1.5 packs of regular kanekalon to make permanent crocheted extensions that were 14-16 inches long for my whole head.
To make them longer, you need more packs of hair.
Going by weight isn’t really the way to go. Sorry I can’t offer you more advice without more detail to your question!