Maker and Black/Dark Crystals

i made archer shoes with basic wis and dc and they came out 1 int

Archer shoes with Int?

Since black/dark crystals only can change your stats 3 points above or below, and since there ¡s apparently no chance for it to fail (change nothing), should I guess that the probabilities to get any change are:

100/6 = 16.6% to all?

that is:
-3 = 16.6%
-2 = 16.6%
-1 = 16.6%
+1 = 16.6%
+2 = 16.6%
+3 = 16.6%

Excuse me if I am wrong I dont have much idea on what im doing, can someone check this? :f6: