Could you tell me how much flaxseed oil would i have to eat each day to get 300mg dha?

Flax: It is not truly known. The reason is that flaxseed oil is actually contains a different omega 3 fatty acid similar to dha and epa, called ala, of which approximately 15% consumed can be converted into dha/epa. The conversion amount is not equivalent to the proportion, therefore exact amounts are difficult. I've seen some estimate that one gets up to 50% of dha from flaxseed oil than fishoil.