One teaspoon is roughly 5 ml. One liquid ounce is 6 teaspoons.
To calculate go by mg of omega 3 fatty acids, you add up the amounts listed for DHA, EPA and DPA (if listed) per serving. DPA is usually not listed, but except for pure menhaden and salmon oil it's not a big factor and can be neglected.
An 80-lb dog would get a maximum of 50 x 80 = 4,000 mg per day.
Say a product contains 800 mg EPA and 500 mg DHA per teaspoon, that's 1,300 mg total.
4,000 divided by 1,300 is 3.077, so you could give up to 3 teaspoons per day.