A silver engraved tool should give +2% success chance but it gives less then this. If success chance is 95 percent with a normal tool it should be 97% with a silver engraved. Either 95+2 = 97 or 95*1.02 = 96.9 If success chance is 91 percent with a normal tool it should be 93% with a silver engraved. Bit it stays at 91%. Either it should be 91+2 = 93% or 91*1.02 = 92.82%. The +5 skill for bronze engraved ones, seems to work. But i do not have tested the other ones.