If you’ve invested in housing cutters as well as shifter cable cutters, it’s always cheaper to just buy bulk lengths of housing and individual cables. If the OP has those tools, then have at it. The advantage to the pre-cut kits is they are ready to go, no cutting tools needed, usually. KCT’ post shows an example where the kit lengths are not going to work, but I can say after 36 years and 20 some odd bikes and many more cases of changing shift cables and housings, I’ve only owned one bike that had the rear derailer cable route across the down tube and down the seat stay, all others (except my electronic bikes) have had the housing stop mounted on top of the drive line chainstay, or were an internal cable routing with a housing stop on the seat stay. In those instances, the precut housings were the correct length and worked just fine.