I don't believe there is a set defined rule, but generally people recommending 200-400 iu of vit E per 1000 mg of fish oil. We don't supplement vitamin E as we believe they get enough of it from their meat and offal.
Fish oil is more easily processed by dogs than flaxseed. Many people supplement with flaxseed and see the results they want, but I prefer to feed my dogs strictly fish oil along with a diet heavy in fish. Evening primrose oil is also a common supplement, but it provides omega 6, which a dog should get more than enough of in kibble, meat, eggs, etc. One should find a good balance between omega 3 and 6, but because omega 6 is already so common in a normal dog diet this is why supplementing omega 3 is recommended.