Technically speaking this method is even more accurate than using a 1:500.000 marked ruler, since your chart isn't a uniform 1:500.000 throughout.
Yep i had that issue around question 15 in my Nav exam when i realised my fuel planing for the trip was quite wrong. lots of re measuring and erasing.. and quiet swearing. My ruler wasn't long enough.
Thats the first time thats happened!!
