I got started in electronics while in elementary school. I had designed and built my first radio by age 13, for which my parents rewarded me with my first slide rule. I forget what it was, but since we were not affluent then, I assume it was a basic one. I used it through highschool, mostly in trigonometry and chemistry (my school did not offer physics separately).
In the Navy, once we finished schools and were assigned to vessels, there wasn't time for calculations. We enlisted technicians repaired equipment failures based on memorizations of symptoms and cures, and some troubleshooting skills. On subs, only officers and quartermasters made calculations during navigation or torpedo attack calculations.
Once discharged after a minimum hitch, I used my GI bill benefits to acquire first an AS and then BS-equivalent training via correspondence courses. I couldn't afford the trip to Ohio for my batchelor's degree conference exam. It was during this time that I acquired the first two rules below.
Then in 1972, HP introduced the HP 35 pocket calculator and slide rules everywhere were relegated to the lower-left desk drawer, used only when the calculator batteries went dead, and less frequently as techniques were forgotten.