If you are tackling a Dredd helmet (comic) I can say from doing the Minty helmets its an a#se to get it right!
The main problem with the helmet is that nearly all the artists draw the helmet far too short nose to back of head...
If you look at the classic 'Gaze into the fist...' pic that's about the only picture where Bolland has drawn a head and then drawn the helmet, rather than drawn a helmet and the face after ;-)
You can see its far 'longer' than a normal sketch of a Judge's helmet...so if you do a 'classic' proportion it looks too long like in the Bolland sketch, so if you then bring the ear to ear up to pro, it ends up far too big....
If you remember these points during the sculpting stage,you should be ok size wise ;-)
Its hard to describe, which is why I sound like I'm waffling.
The helmet Greg Staples wears in the Judge Minty film was my 'final' version ;-)