Specifying when to announce in time rather than distance would seem to make more sense to me; “10 miles out” at 60kts is 10 mins, at 120kts, 5mins, and in a busy pattern everyone will have forgotten the announcement by then and the arrangement of traffic in the local pattern completely changed. May be “three minutes out”, which would be about right for a typical FAF at 90kts, and is probably enough time for the information to be actionable. I wonder why the FAA picked 10 miles?