Okay, so using Hidden-Streets.com and its damage calculator, I took the stats of a level 15 Thief.
LEVEL 15
04 STR
30 DEX
04 INT
57 LUK
We shall say he has a Level 15 Field Dagger equipped.
Field Dagger has a Weapon Attack of 30.
With those stats:
Minimum Damage: 15
Maximum Damage: 71
According to Hidden-Street.
Now, let us up the Weapon Attack by 1.
04 STR
30 DEX
04 INT
57 LUK
31 WA
Minimum Damage: 16
Maximum Damage: 74
Now, let us lower the WA back to 30, and add to LUK until we hit numbers approx. of what you just read.
58 LUK
Minimum Damage: 15
Maximum Damage: 72
59 LUK
Minimum Damage: 15
Maximum Damage: 73
60 LUK
Minimum Damage: 16
Maximum Damage: 75
So, it took 3 LUK to match 1 WA, but then again, the Max Damage is one higher.
As scientists do, they repeat experiments.
Well continue from the current stat of LUK, even though a level 15 would not have 60 LUK, unless he didn't have 30 DEX, or had scrolled equips.
04 STR
30 DEX
04 INT
60 LUK
30 WA
Minimum Damage: 16
Maximum Damage: 75
31 WA
Minimum Damage: 16
Maximum Damage: 77
Now, drop back down to 30 WA, and up LUK by 1.
61 LUK
30 WA
Minimum Damage: 16
Maximum Damage: 76 (Max increased by 1, with 1 more LUK)
62 LUK
30 WA
Minimum Damage: 16
Maximum Damage: 77 (Increased another 1, with 1 LUK, matching exactly what 60 LUK, 31 WA was)
Took 2 LUK to match 1 WA.
So it's different.
Scientists would continue experimentation, but I don't feel like creating a larger page stretcher.
We can say 2~3 LUK = 1 Weapon Attack.
The Average from this result would be 2.5 = 1.
Of course, I have forgotten that at one time, the Max Damage was one higher, but I'm to lazy to test this further.