Helmet Repositioning
One of the problems of various helmets is that the helmet's flare seems to stick out, revealing the back of the neck. It is often argued that in such cases the "dome" is flawed and that the flare's angle should be steeper. Some fans have even added an extra inch or two of material to lengthen the flange. On one hand there are those who argue that a larger dome was indeed used in the movies, and on the other hand, some have recreated the same look without adding a single inch to the flange of the helmet.
After studying screen shots and hearing both sides of the case, I think the truth is somewhere in balance. Here is my take on this.
One cannot make a generality because Vader props differred from film to film.
In Episode IV ANH, the prop facemask did not have a receiver, so the way the helmet sat on the mask was different than in Episide V TESB. In ANH, the dome's front rim overshadowed the eyes, and the center widow's peak just barely covered the top cleft of the bridge of the nose. In TESB, there was no widow's peak, and the helmet seemed to be worn higher, and now you see Vader's eyebrows when you look directly at it.
Even though in ANH the eyes are overshadowed, and there appears to be a 1 cm gap between the widow's peak and the bridge of the nose, if the cameraman (in a seated position) zoomed in tight at Vader, you could still see his eyebrows. In other words, looking up at Vader you always saw his eyebrows regardless of dome positioning. That 1cm space is vital to allow that.
The most iconic shots of Vader were those where he stared into the air, and even though the face mask was devoid of any changes of expression, you almost saw the solemn depth of the soul of the man behind the iron mask.
When the helmet (collectively the facemask and helmet) are vertically tilted up, the flange on the back of the helmet rotates downward, covering the back of the wearer's neck.
There are many such scenes where Vader seems to look upwards like that. Also, many shots of Vader were done where the camera operator was in a seated position, and the movie camera was 15-20 feet away, zoomed in (to reduce perspective distortion). If you do the home photography thing and attempt the same angle only 2 feet away, you get the same effect as many eBay pictures which is a big facemask but a tiny dome in proportion.
(Above: Left, an already somewhat modified Rubie's helmet. Facemask and helmet are roughly at the same angle as an unmodified stock Rubie's. Right, primed but not quite yet ready modified Rubie's photographed at a slightly different angle but with a different dome position.)
In the shot on the left, notice that the mouth wall is almost aligned vertically. At this angle, the tusk tubes are at a slight angle, not quite parallel with the ground. Notice the rim of the helmet is almost purely horizontal and in line with the aluminum tusk and the tusk tubes. This is Vader on a bad day; there's a draft down the back of his neck. Vader does not wear a scarf!
To the right (this isn't a perfect apples-to-apples comparison as it's photographed at a different vertical angle. My apologies) the facemask is recessed, not just inward but upwards a little. Play with the mounting angle that works for you. The dome is vertically tilted back a little. However, in conjunction with the facemask being recessed, it doesn't look silly. Here, the flange covers the back of the neck, leaving a very minor gap as opposed to the left photo.
Compare the one on the right with the "Tantive IV" shot below.
What fascinates me about this shot is that the chest armor is worn very naturally and at a natural angle. Observe the side profile of David Prowse's body. Notice the flange of the helmet might barely touch his trapezius, yet Vader does not look like he's shrugging. Also notice the forward tilt or vertical tilt of the chestplate. Also the facemask mouth profile is almost a vertical line here. If you drew a horizontal line to the right of the tusks, the flange is past that line -- and I've done this purely by repositioning the helmet, not adding any length to it.
But there are exceptions to every observation.
Here in ESB, at this angle, the flange is almost in-line with the tusk tubes. However, Prowse's well-developed trapezium may be the reason why there appears to be only a 1.5" distance from the rim of the helmet to his cloak.
And now he turns...
Notice that Vader is looking straight on -- not at the ceiling like he does iconically. This is a particularly interesting shot because some fans would swear the dome was too short! I think the cameraman just has to be careful to not photograph into the back of his neck, and to make sure that if Vader's back is to the camera that he should be looking up at the ceiling a bit!