Just like we saved the starting height of the chest, we need the starting rotation of the chest lid.
We could do this in three steps, by setting the x, y, and z values, or we could use the handy method copy, which sets all three based on the values in a different Euler.
Remember, this is the same as:
this.startingRotation.x = this.entity.localTransform.rotation.x
this.startingRotation.y = this.entity.localTransform.rotation.y
this.startingRotation.z = this.entity.localTransform.rotation.z