What is the proper method for an instructor to calibrate a student's following distance on rural roads and highways?

1. Tell the student to stay 2 meters behind the truck ahead to draft for fuel economy
2. Measure distance by looking at the tire air valves of the leading car
3. Have the student pick a stationary object ahead (e.g. a signpost or bridge), count 'one thousand and one, one thousand and two, one thousand and three' when the leading car passes it, and ensure their own car does not reach the object before finishing the count
4. Follow as closely as possible to prevent other cars from cutting in