Rhumb line track calculation [Archive] - PPRuNe Forums
If you work out the N/S distance, it comes to 210nm; if you work out the East/West Departure distance at 45N it comes to 212nm-ish. You therefore have an approximate right angled isosceles triangle where the right angle is at 48.5N 15W.
tan(angle) = opposite/adjacent
opp= 210nm and adj = 212nm
Now invert the tan function, you get...
(angle) =tan^(-1) (210/212) = 45 degrees
Add this to the 270 degees and you get 315 degrees precisely!