This is all provin' very stimulatin' to the few remaining grey cells ...
@CrossRoads, thank you, but I am not concerned with accuracy greater than you get from millis(). This is just intended as a simple learning device for newbies.
@PeterH and @CodingBadly, I hope you will have realized that my only objection to doing it "properly" was the risk that explaining the change would so confuse a newbie that all value from the demo code would be lost. I think I have now figured out a simple way to link the "+= interval" version to the "= currentMillis" version that's in Blink-Without-Delay without the need for a complex explanation.
I will modify my example and maybe post the updated version later today if I feel the change will be sufficiently unobtrusive. Edit to add ... I have now updated the code attached to the original post and the code in Reply #1.
@CrossRoads, I don't understand your use of the term "mile marker".
...R