setting up LDO with variable output current (+diagram)
The dropout voltage is the minimum input to output voltage that allows the regulator to still regulate. Knowing the output voltage, that gives you the minimum input voltage for regulation at the current you want.
Note power supply rejection ratio (PSRR) of LDO decreases as they go near dropout, so YMMV. But, PSRR is not specified in the datasheet anyway.
You don't need a linear regulator for your LEDs. Just use the output of the switcher directly. If they are WS2812B they already regulate their current. If they're just LEDs, you can wire them in series and use 12V directly. It's much simpler.
Unless the micro is doing sensitive analog things you also don't need a LDO for the micro, it'll run just fine with a bit of switching noise on VCC.
However, I hope you don't plan to use the counterfeit cheapo &quot;LM2596&quot; modules?...