A tip for doing the left reverse is to get the pupil to turn the left door mirror down slightly so they can see the kerbstones. Tell them to keep the kerbstones about 2 fists away from the kerb as all times and adjust the steering in small amounts each time to keep the distance the same.

By using this measure they can visualise what 2 fists look like and they will be able to keep the car at a reasonable distance from the kerb without touching it. Stress that they should be no closer than 1 fist but no further than 3 fists and that should work.
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