im about 5'11", i have mine at 26.4CM (assembled height) the scale factor is something completely different, and for the newhelm.pdo(my custom halo2 model) you can use the original scaling method posted in the sticky. also on my halo 2 model, you will have to modify the neck opening, it was something i did not take into consideration during the design, hopefully my new model will be better, im not done building it yet, but i will find out soon enough.
for my smooth helmet you have to take the halo 2 helmet, scale it and find out how big in centimeters it is, then use that size (in CM) on the halo 3 helmet, once you do that it will generate a new scale you can use for the rest of the halo 3 armor.