BEagle: "12. The hardest sum next! Time = Distance / Speed! Either mentally or on the back of the checklist. Write it down, then lookout."
Avoid the random number generator known as "hard sums" by simply marking the track line at intervals of 1/10 of groundspeed (equivalent to 6 minutes) and adding-up the 6s (e.g. groundspeed 140 equates to 14 miles in 6 minutes).
HFD