Looking for advice on insulating a sloping ceiling

[Westerco]

I'm planning to install internal insulation in one of my bedrooms which has a dormer window and sloping ceilings either side of the window. The walls have been stripped internally and I'm just about to order the insulation.

Both the walls and ceiling will be getting insulated with flexible wood fibre insulation between the rafters, but I'm not sure if I need to put the insulation along the whole length of the rafter to beyond where the ceiling meets the wall, or just extend it a little beyond where the roof meets the wall.

I would appreciate any advice you could give me on this.

Thanks,