geometrically the highest latitude comes out to be around 87N
Correct, but it's not asking for the scale to be used at the highest Latitude - it's asking where is an approximate scale to be used to get an approximate distance. (Average Latitude is better - you went from 70N - 80N - 87N - 80N - 70N. 4 equal sections with average 37/4 degrees above 70 N = 79.25N)