I would say it should go with the level difference between your crafting level and the level of the craft you doing eg: something like +0.2% for each level difference. So higher you are higher are the chance to critical. Something like temparature of environmental factor could be a nice modifier to put in. But first i would like to hear that eventually they might add this into the game
. I would regret a crafting system like wow that end-game wise, you got almost nothing really useful to make money with.