If I use an item such as a Spell Tag on Dusknoir, how much more power will Shadow Sneak have gained?
40 x 1.5 = 60 (this is Base Power x STAB), how should this look if I gave him the Spell Tag?
Well Spell Tag boosts the power of Ghost type moves by 20% so:
Shadow Sneak = 40 Base Power x 1.5 (STAB) = 60 x 1.2 (Spell Tag) = 72 Base Power.
Hope I helped :)