Great question — answering as the UphillTech team:
Try an egg timer that measures temperature. Here's an example (that I don't necessarily endorse): Egg Perfect Timer The idea is, the device supposedly absorbs heat similarly to an egg, and gives a visual signal of the level of heat absorption. If the water is colder than typical, because you're at a higher altitude, the time it takes to heat the eggs and egg timer will lengthen, but the amount of heat the eggs and egg timer need remains consistent. I had one very similar (perhaps that same brand) when I was much younger. I remember it being hard to read, but I think it was accurate.
Full answer by Juhasz: https://cooking.stackexchange.com/a/101186 (CC BY-SA 4.0).
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