Write and keep open on you desktop a sketch for the same kind of strip that just
turns off way more LEDs than you have
upload it whilst thinking and tinkering. Save your eyes, save power &c.
The same small sketch is good for finding the end of a strip. I use strips of various lengths and got sick of counting… I’ll leave it to you to write the hack that can rapidly help you determine how many are actually present.
It’s good also for testing, again your own opinion will inform what kind of functionality you want for that kind of thing.
Being a small sketch, it will whistle right up and do its thing. All set for the real sketch when it’s ready fir another go.
HTH
a7