OK @ketre , The Fenders as they say in some parts, wheel arches in others, are not quite complete, as to say they need to be extended in the x direction in towards the cab so I would ignore them as a reference for now I was going to wait and use the cab as reference. But your right the cab has got an awkward shape, it probably is shaped around the fenders, but I wouldn’t worry about trying to do that. I would suggest at the point where the angle changes just above the fenders, it should be more or less vertical or even angled inwards. See the blue lines I have added to this Image. It wont go any lower than this as that is approximate height of the chassis. but I would go with this for now.
