How to sequence channels - mapping or programmatically?

I feel like I should know this as it seems easy, but I can’t seem to frame the question correctly.

I am trying to sequence together multiple channels on a serial expander to form a single long strip. so ch0:0 to a, ch1:a+1 to b, ch2:b+1 to c, ch3:c+1 to d. WS2812B, 150 leds each, power injected into each strip, common ground.

I have to do some of this planning away from the actual physical strips/pixelblaze (still >really< wanting an emulator!), so - is this best done as a circle map that assumes total pixel count is a:d? or a matrix map with a zig-zag function?

Or can I determine which channel is being rendered programmatically and just maintain index position for each in turn? Can I determine anything about which channel is being addressed at all?