There's a specific dose of EPA/DHA that's required for therapeutic effect. Adding less than that will only add fat to the diet -- and why would anyone bother to use fish oil without it being therapeutic. It's the same for people.
So for my Dog daisy who is 8 lbs takes one Nordic natural capsule every day. I pierce it with a knife and squeeze it on her food, tossing the plastic capsule. This is the only fat in Daisy's home cooked diet other than the small amounts found in her protein sources.
If you use a different brand, you have to calculate the dose of EPA/DHA.
Since I am not a nutritionist I don't feel comfortable telling people what they should add. However, if anyone would like to visit the "ask the nutritionist" section of
PetDiets - VNC putting pets' health first!, you can search the word omega and there's a post where she advises someone of the dose. I do not know if her advice would apply to other dogs beyond the one she posted, so if anyone wants to know, they should ask her directly, especially those of you who are clients already.