@CodingBadly, I'm interested but I don't understand exactly what you mean by "The average interval will be slightly to significantly more than onBoardLedInterval"
My plan was/is to use a single instant in time (as captured in currentMillis) as the reference point for the flashing of all of the leds so that their timing can't drift apart.
I can't figure out what effect your code would have.
...R