Do you assume the 7.5 limit of armor weight is true of bows along with thrown and crossbow? I was hearing an estimate of a 5 weight limit (which seems kinda low to me, since a helmet can rapidly gain weight due to the multiplying factor), but 7.5 would give me more latitude.
As far as i heard the limit for archery was ~6. I'm pretty shure it WAS. I dunno if it's still the same limit.
I would advise max. a Studded Leather Coat for archery. Most of the times a helmet is never needed, if anyone gets a headshot, oder hits you in the face with his weapon you will be dead anyways, a headarmor of ~15-20 wont save you.