I took my old Haws watering can that was a gift from my father about 20 years ago, filled it with water and fish fertilizer. I watered it into the veggie garden and the greenhouse bed. I also gave a good shot of it to the Rhubarb too. I used 80 ml to two gallons of water.
Update: I worked the math out exactly and now I use 100 ml of fish fertilizer in the 8.8 litre (which is about two gallons) Haws watering can. I use a glass measuring cup which makes it really easy!