How do you calculate the true bearing from a station (QTE)?
You are quite close to the answer, you just made a mistake with the sign of east compass deviation, hence underestimated the true heading by 6°. The actual angles are like this:

  

The compass deviation is given as +3° while it should be given as 3° east, like the variation. It could be misleading.

Exactly as you then developed, the unknown yellow angle is equal to:


True heading + (relative bearing - 180)
147 + (307 - 180) = 147 + 127 = 274°