Lower level gear is much harder to predict, and unfortunately there is no equation you can use to tell how much of a weapon's damage will go into skill damage. It is somewhat random but in general higher damage weapons add more than lower damage weapons, particularly if you are looking at gear for the same class from the same campaign. There are a number of notable exceptions though. The otherwise excellent shock lance for example only contributes a tiny bit to skill damage. Level 35 two hand swords seem to add more to skill damage than 2-handers from the next two higher campaigns, etc., etc. Like I said the higher damage -> higher skill damage thing is a general trend not a rule. You have to try stuff on yourself and compare. At higher levels though it does seem to be more predictable that higher damage leads to higher skill damage, but even then you just have to try stuff out