A very rough guide, going with the primary damage type, is:
Dagger - d3
Short spear - d4
Sword/shortbow - d6
Longbow/mace/hammer/halberd/spear - d8
Axe/heavy crossbow/flintlock - d10
Musket/rifle/pepperbox - d12
Again, this is pretty variable; if you sneak up behind someone and slit their throat, it sure ain't gonna be d3 damage. If you ram your sword into someone's lungs, it's gonna hurt more than d6.
The existing damage types are: Slash, Pierce, Bludgeon, Ballistic, Fire, Cold, Electrical, Acid, Holy, Unholy, Death, Magical, Auric/Psychic, and Unblockable. Obviously, Unblockable damage can't be prevented.
* Bludgeon (usually) doesn’t penetrate the skin, but it can break bones or noses. In real life, head hits frequently cause concussion, brain damage, or death. In cinematic stories, they’re a great way to render someone unconscious. Examples include clubs or fists, and concussive damage (explosions) are also lumped in this category in general.
* Slashing cuts tissue and can sever blood vessels or nerves. Examples include knives, swords, and saws. Rule of thumb here is that cuts bleed. Even if they have partly healed, cuts can re-open under stress. Bleeding wounds provide you with a terrific dramatic device to increase tension, as they add that “clock-is-ticking” element to the Scene. Can the character succeed before he bleeds to death?
* Piercing comes from wounds that penetrate body tissue deeply. Punctures are hard to heal. Further, if they occur on the torso or the head, can damage internal organs as well. Examples of puncturing attacks include, arrows, knife/sword thrusts, and axes. Ballistic is a subtype of this, and is usually more dramatic.
* Burns results from exposure to extreme heat, cold, some electrical damage, acid. Burns are extremely painful and take a long time to heal. Scars can be incredibly disfiguring.
* Special damage is stuff that doesn’t fall into the other categories. Effects are highly variable, from harmless to fatal. Examples include tranquilizer drugs, poison, nerve gas, electrical damage, drowning, and radiation.
In general, armors defend against a few types of damage, and protect against some better than others. A breastplate is good against slashing and piercing (4 damage absorbed/soaked), of less use against bludgeoning and ballistic (2 soak), and does fuck-all against anything else. A Bracelet of the Elements provides 4 soak against Fire/Cold/Electric, and stops nothing of physical damage. So on, so forth.
Define "measures of success."