Why re-mark your map with 6 minute markers if you're running fast/slow? I only use the original markers. It's no problem working out divergence from planned times so planned ETA can be adjusted accordingly. I disagree that the marks are confusing if groundspeed diverges from the plan. If the student is staying with the plot they'll be fine; if they're not then they're burggered anyway.
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0 to 2000ft in 10 minutes