C 15 descending hills question

There's an old rule of using your service brake only 25% of the time downhill. I do heavy haul in Washington, Oregon and Canada, for me personally ill drop 2 full gears when going over a hill. If there's no hill I drop 2 full gears from whatever gear I'm in at 65 so if I'm in 8th I'll drop to 6th. If you press the brakes for 3 seconds and drop 5mph according to the 25% rule if it goes up more than 5mph in 12 seconds then drop another gear