I advocate the simple percentage method for MDR when calculating the time for an unplanned diversion as has been described.
But for pre-flight planning, I expect the student to use a computer of some form, not MDR.
And as for the organisation which teaches people to use still air planning at medium level if the wind is 'less than about 10 kts'.... That's just laziness; fair enough if the wind is 'light and variable', but if you start out with such a 10-12% error in groundspeed, then on a typical 50 mile leg you'll be outside the PPL Skill Test limits before even taking off.