[Siven]

According to http://www.moonlight.gr.jp/~el/ffxi/ab_calc.html.en it boils down to:

Base Dmg + Weapon Dmg x Dmg Rate

where

Base Dmg = your STR - mob VIT
Weapon Dmg = 3 + floor(Combat Skill / 10) + Weapon's Base Dmg (whatever it says in the description)
Dmg Rate = (your ATK / mob DEF) within restrictions related to mob's comparative DEF.

There're some additional modifers for calculating criticals with dmg rate, but we can discount them for now. Now let's have some fun with the numbers.

A Galka DRK at lv42 has 45 STR and about 189 ATK on average, using, for example, a D64 scythe. Combat skill is capped at 129.

We've no info about mob's VIT/DEF, so as substitute for an IT mob I'll use a Galka PLD who's lv50 and have suitable armor. His VIT is then 58, with 155 DEF (126 from gear + 58 / 2). Realistically this might be higher for mobs. Now plug in all the numbers...

Base Dmg = 45 - 58 = -13
Weapon Dmg = 3 + floor(129/10) + 64 = 79
Dmg Rate = 189 / 155 = 1.22

Therefore total damage is -13 + 1.22 x 79 = 83, and 96 if we discount the nonpositive term. However I should point out that because the PLD will likely con IT with high DEF to the DRK, the DRK's dmg rate will be limited to 0.5 max. So realistically the total damage is thus -13 + 0.5 x 79 = 27 to 40, if we treat -13 as zero.

P.S. More on the Dmg Rate caps: if mob cons high DEF it's 0 to 0.5; normal DEF, 0.5 to 1.5; low DEF, 1.5 to 2.0. So in the above situation, if the PLD conned low DEF to the DRK, the damage jumps to -13 + 2 x 79 = 145, and 158 (with -13 treated as 0).