Book Library lit by Fadecandy controlled LEDs. HTML programming Q

I’m pretty chuffed at how my Library project is coming along, but I need a bit of help with some programming. Due to the many different length runs of LEDs, (I just counted them, 44 sections) there are numerous odd length strips as well as data injection at various places in the middle of runs, although the overall result is linear. For programming I need to find out where each section starts and stops so that i can write the fcserver config file correctly, as well as have a chance at getting the programming sorted.

I am thinking of an HTML page where I can dial in a number using a scroll button or the like, and light the LED that corresponds to that position.
Firstly i would use it with the default config file to check the exact lengths of the strips on each of four fadecandies so that I can write the config.json file, then use it with that config file to define the various sections.

I’ve check out the HTML and other examples in the fadecandy documentation, and i think the answer is in there somewhere, I just need a bit of a push in the right direction. Thanks for any help

I know, I know, I could (with difficulty) just count the LEDs on each strip, but what fun is that.