A couple times a month maybe. Usually I just guess.
Years ago, I was making an LC circuit for a loudspeaker (on my lunch hour, of course). I was carrying a coil, probably 6" diameter of magnet wires, about 4 turns to go test it, from across the lab the savant engineer asked "what frequency are you after?" I told him, he said I need 2 more turns. Yeah, I calculated this I'll trust the results of the test, thank you.
I needed 2 more turns.