I'm recalling my previous thoughts on this too:
https://budgetlightforum.com/t/-/46073/363
"2S are also the only ones I see as potentially truly calibration free, assuming the LDO has good specs. inverted, or "Internal" read, divider-less 1S still uses the 1.1V internal reference (so does the normal method not referred to as "internal"), but on 2S you can reference the LDO. "
Yep, right, using
#define REFERENCE_DIVIDER_READS_TO_VCCAs my included LDO configs do... gives you a more stable voltage reference than any 1S build. OTSM builds require using the max range on pin7 (voltage sense pin) to stay above the pin change spec at low battery voltage* (pin change triggers an interrupt). So if 1% resistors are chosen correctly for R1/R2, there should be no calibration needed at all for LDO builds.
* From the manual:
For a multi-cell(ex: 2S) light, Vcc will be powered by a regulator and cannot be used to read battery voltage. If a 3.0V LDO is used, then the highest shutdown trigger level on the voltage pin is 1.8V or0.6*3.0. If we reference voltage reads to Vcc the max we can read is is 3.0V. Conveniently 2.5V is 0.6* 4.2V so a typical battery operating range just barely fits in the available headroom. Resistors must be selected in 1% tolerance so that max voltage corresponds exactly to 3.0V** or maybe 1 or 2 %less. The voltage should be read from the divider and the option to reference divider-reads to Vcc should be set, since these voltages are necessarily above the 1.1V internal reference.
** Actually 3.0V minus the diode voltage, if any, to be exact. Details on that are in the manual too, but it's a small correction.