Consider triangle A (45ºN010ºW), B (48º30'N015ºW), C (45ºN015ºW).
You have calculated that side AC is 210nm.
Side BC is also 210nm: change of latitude = (3x60) + 30 = 210nm.
It is an isosceles triangle and angles CBA and BAC are equal.
Angle BCA is 90º (meridian crossing parallel of latitude) so angles CBA and BAC must be 45º.
The track from A to B is 270º + angle BAC = 270º + 45º = 315ºT
Rhumb
lines are straight lines on a Mercator:
015ºW 010ºW
48º30’N
45º
3x60 + 30
= 210nm
45º00’N 90º 45º Track = 270º + 45º
= 315ºT
Departure = 5 x 60 x cos 45º
= 212nm