shed some light on how are we are suppose to keep that aircraft within the allocated protected area
That comes under the title of training and proficiency which comes down to sim practice .. if you are fortunate enough to have an employer who puts dollars into extra training in the box.
Forget the eyeball .. gradients are too small to estimate and, with the body angles, how do you see over the nose in any case ?
Other boxes can be useful .. eg, backcourse LLZ tracking (a very useful sim training exercise to get the stick and rudder skills up ... try a min weight, aft CG, failure during the rotation with a requirement to so track) ... and, these days, preprogrammed FMS escapes may be useful.
I think you're over thinking this.
I don't think so at all. There be dragons outside the splays
fly the Boeing profile,
However, the Boeing profile doesn't put you in the middle of the splay, that's a pilot task and responsibility.
Anyone taking measurements in this circumstance can get stuffed.
Of course they can.
However, the aforementioned dragons can be right on the splays. It is not at all uncommon to run the splay up the side of the mountain so that the sums work out. If Captain Speaking allows his/her aeroplane to diverge too much then it's all over, Red Rover.