I'm familiar with those rules of thumb and I'm sure there are some folks who will thank you for that tip.
I'm trying to help answer the question of how did the Analog TAS indicator work which is still an unknown to me. Thanks for the tips.
Airspeed indicator
A device that computes and displays speed of an aircraft relative to the air mass in which the aircraft is flying. The commonest type is the indicated airspeed meter, which measures differential pressure between the total ram pressure from the Pitot system and the total static pressure; it then converts this difference into units of speed (mi/h or knots) under standard conditions. Although the indicated values are incorrect above zero altitude, the relationship to the aircraft handling remains essentially unchanged, thus providing a measure of the flyability of the aircraft.
True airspeed indicators are similar but include a more complex mechanism that also senses both the absolute pressure and temperature, and compensates for the change of density of the air mass, thus obtaining true airspeed. This indication is of value in computing course information.
Hope this helps