My parents never used a timer. They lifted the egg out of the water and observed how quickly the water evaporated from it. (This would be a measure of the egg's temperature.)
If you don't mind using a dozen eggs to find out, try lifting one up, timing how long it takes to dry, then cracking it open, do another, etc etc until you know what you're looking for. At first you might actually need to time this drying process but I am sure it's possible to learn "hm, yes, this egg needs some more time" vs "yup, just how I like it" within a few weeks of normal egg consumption.
(Found an answer (first one aqui) from a uk person stating that 10 seconds is the time for soft boiled - personally I would want to test to discover the time, but ok, and the person mentions living at altitude also.)