John Grabowski wrote:

4" of slack will give you 2" of each end inside of a junction box. Pull out
the last 10' and run a new 11' or longer cable. The holes in the joists are
already drilled. It's not that difficult a job to pull out an old cable and
install a new one. You can cut the old one at different intervals to
facilitate its removal.

4" of slack plus the wire that would be inside the box if there were no
slack - approximately another 3"-4" on a ceiling box. Should be more
than enough to pigtail off from that.

R