My DF game uses the following armor rules:
- Basic Set armor
- Layering (p. B286) doesn't give a -1 to DX on the head. Although it's not concealable you can wear a mail coif under rigid armor. You can wear three layers on the head, in specific combinations (mail or cloth under a pot helm under either a greathelm or barrel helm). You can't wear more than one "cap" or more than one flexible layer. So no cloth cap under mail coif under pot helm under greathelm combinations. Because Cloth Cap is just 1 DR, maximum Fortify is to +1 (DR 2), and doesn't protect the neck or face, it's generally a poor choice and I left it off the lists. Also, you can't put a face mask under a greathelm, and yes, I've been asked - no doubling up face protection!
- Fortify and Lighten can and do stack.
So this leaves a few really useful combinations of head armor. I figured I'd list them out for my players. This isn't exhaustive, it's just what I see as the better combinations.
Kings don't always worry about neck protection, but sometimes layer armor.
Leather Helm - DR 2, $20, 0.5# (skull, face)
+Mail Coif - DR 6/4*, $75, 4.5# (adds skull, neck protection)
Legionairy Helm - DR 4, $150, 6# (skull, face)
+Mail Coif - DR 8/4*, $205, 10# (adds skull, neck protection)
Pot Helm - DR 4, $100, 5# (skull)
+Mail Coif - DR 8/4*, $155, 9# (adds skull, neck protection)
+Face Mask - DR 4, $200, 7# (adds face protection)
+both - DR 8/4*, $255, 11#
Great Helm - DR 7, $340, 10# (skull, face, neck)
+Mail Coif - DR 11/9*, $395, 14# (adds skull, neck protection)
+Pot Helm - DR 11, $440, 15# (adds skull protection)
+both - DR 15/13*, $495, 19# (adds skull, neck protection)
DR listed is for skull, without counting any natural DR (usually 2). Check protection for individual locations when purchasing! And double-check the rules on p. B284. This is just a handy guide post - stuff I left off isn't "invalid" just not listed. Note  especially.
So yeah, potentially, you can blow a lot of weight on your head and get DR 20/18 on the skull (greathelm, mail coif, pot helm, all with Fortify +1, and skull), blow even more and go crazy-go-nuts (make the armor Dwarven), etc. I've calculated the maximum as 38, counting the skull's DR, assuming you allow Dwarven on both pot helms and great helms and using Elven mail. That'll be enough to stop an attack up to 6d cold, and most 7d attacks . . . if you can afford the cost and weight! If only you weren't more vulnerable everyone else thanks to inability to stack up as much DR on your torso, limbs, and extremities . . .
Editing Later: My players all know this, but layered armor is bought as a set. That is, you need to get upper layers tailored specifically to go over another set. If you don't, the penalties for mismatching are doubled (and penalties on the head are -1 to DX and Per.) Wearing an "over" layer without the "under" layer is a -1, as if you'd stacked armor anyway. I realized this might not be clear - you can't just snag a random pot helm and great helm and stack them, but you can go out and get an armorer to fit you out with a coif under a pot helm with a great helm that fits nicely over them all.