Gally2
Also, do pilots have to do this for every flight in their heads (or on paper) or is it all on computer now
Used to be taught in the military as part of pilot nav when you sometimes had to plan or replan something with minimum time/kit, TBH I can't remember where it sits in the civvie syllabus/ATPL, but the same maths certainly has it's uses in the civvie world (it's rare these days but e.g. flying "raw data"/"non computer" approaches where you need to work out a heading to fly a published track towards or away from a beacon or down an ILS, working out drift corrections for holding patterns, etc.).
Just to amplify slightly on the answer you got: "Max drift" is literally the maximum drift that a given wind can produce ( i..e when it is on the beam), It's calculated by the formula captainsmiffy describes (windspeed/TAS in miles per minute), and then for a wind that is not "on the beam" is corrected to give actual drift using the described "clock code".
captainsmiffy - Thank you very kindly for offering the full explanation......I'm rather glad you saved me the hard work and possible embarrasment - my "A2" ticket is well past it's sell by date....