It's usually one tablespoon to a gallon. I had to do an on-line conversion, and 500 ml isn't close to a gallon. So I would definitely use more water than that. You can use the tank water for the before and after container, but I would prepare a different container for the actual bath. Usually epsom salt baths should last no longer than 15 minutes. Make sure that the salts are dissolved in the water before putting the fish into the bath. If the fish is stressed as soon as you put it in the bath, or if you see the fish defecate, take the fish out immediately. That is a definite sign that the bath is too strong.
If you aren't also using an antibiotic, you might want to use one along with the baths. I would do a bath twice a day when treating dropsy. It is a very hard condition to deal with, so I wish you luck!
I see that you have posted a few times about this.
I would have the container with the tank water and the fish waiting on the side.
Get the bath ready, and make sure that the temp matches the temp of the tank water. When the bath is prepared, put the fish into the bath. Don't leave it in the bath any longer than 15 minutes. After the 15 minutes is up, take the fish out, and put it back into the tank water container, so it can recuperate from the bath. After a few minutes, put it back into the big tank.
Repeat twice a day.
If you can get your hands on a decent, strong antibiotic, then dose your fish according to the instructions. In the states, many use
Kanaplex (kanamycin), which is a broad range antibiotic that can treat gram negative and gram positive bacterial infections.
If you are absolutely certain that your fish has dropsy, there are some things that you can do to help. Drop the water level so that the fish doesn't have as far to swim to get air would be an example. Have something for the fish to lay on, like a
betta leaf that attaches to the side of the tank. Keep the tank warm. Add Indian
almond leaves to your tank if you don't already have some in the water. These leaves release tannins that help sooth the fish, and they promote healing.
Again, I hope it all works out for you. I have been in your shoes before, a few times. I wish you luck!!!!