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Protection List
Accessories can't be protection reforged, that's -8 prot. However, protection reforge's cap is lvl5, not lvl4. That's +5 prot, resulting in total -3 prot from your equations.
Well you might be partly right. Just looking at the jp wiki page for reforges, http://mabinogi.wikiwiki.jp/pukiwiki.php?%C1%F5%C8%F7%2F%BA%D9%B9%A9, it says protection reforge can go on accessories and the cap is level 4.
However, looking at the kr gameabout page for reforges, http://mabinogi.gameabout.com/guide/?id=776&cont=5301, it looks like accessories have a protection level cap of level 3, head gear has a protection level cap of 4, and boots, gloves, shield, and body have a protection cap of 5.
So if I went with the kr gameabout data I guess it would work out to be -2 for accessories and +4 for other non headgear for a total of +2.