What's wrong with the good old 10nm thumb? Or 60nm hand span?
I don't know about the rest of you, but I can only fly one aircraft at any one time. If it's an aircraft which I don't fly regularly (and which flies at a speed that's different to the aircraft I fly regularly) it doesn't take much effort, beforehand, to work out and memorise the times for 10nm and 60nm. No point doing it much more accurately than that - since you're not taking the wind into account you won't get an accurate figure anyway.
FFF
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