The distortions due to the projection which Richard correctly describes should be less than 1%. When the UK CAA used these charts 10+ years ago the bigger problem was the distortion caused by photocopying, hence you use a meridian close to or at the centre of track as described above, working on the idea that the meridian and the marked track will have been distorted by similar amounts. In real life, of course, you would never use photocopied charts.