The drift is the heading error.
It is due to two things :
- earth rotation
- moving along a parallel of latitude
You've got here :
1.7h * 15°/h = 25.5°
8° of longitude variation
Multiply that by sin(meanlat) which is about 0.7
You'll find something very close to -24°
Just take a few minutes to imagine the situation and confirm that both signs are negative.