How are the difficulties of called shots determined in Dwarf Fortress adventure mode?

In Dwarf Fortress adventure mode called shots (aimed attacks as they're called in the game), have two modifiers.

Difficulty (Chance to hit)
- Easy Strike
- Tricky Strike
- Difficult Strike
Squareness (Damage Modifier)
- Can only graze
- Square
- Very square

How are these things determined, and can they be manipulated to give me an:

Head Easy Strike, Very Square

Weapon skill is the best way to increase this. I have a legendary +10 swordmaster, and he can make these hits like it's nothing.

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