Very cool idea. Especially for cranking out a quick and dirty paint detail.
So just to make sure i got it:
1 Apply all "solid" base coats
2 Apply decals
3 Paint over decals
4 Any final touch up of base coats
And as for the type of helmet, I guess, do some paper printouts to adjust for sizes before printing the decals.
Finally, is your painting technique just with a brush?