The math and steps involved to determine gallons utilizing salt are fairly simple:
1.) Measure the salinity percentage or parts per thousand of the pond PRIOR to adding any salt and note that reading if it is above zero.
2) Based on your guess of your gallons, add .83 pounds of salt for every 100 gallons of pond water, and give the salt at least four hours to mix thru the system.
3)Now measure the salt content again. If you had any salinity on your first reading you did before adding the salt, you will need to subtract that reading from the second one. This will give you the percentage the salinity changed. If the first reading was zero, simply use the second reading.
4)Now multiply the pounds of salt you added to the pond by the constant of 12.
5 Now divide the answer of #4 by the percentage salinity or salinity change. The answer here, gives you the volume of your pond in gallons. Do not be surprised if the true volume is well below what you originally guessed, because most folks are from 25% to 50% to high on their estimates when they calculated the volume by measuring the pond.
NOTE: The type of salt used should be 99.9% sodium chloride, with no additives or preservatives, and especially do not use salt containing YPS. (Yellow Prussiate of Soda) I use water softener salt, which is very inexpensive, but be aware that not all brands meet these specifications. So check the label first. You can find such salt at most of the home improvement super stores. It is also important that you pre-dissolve the salt before adding it to the water. If un-dissolved salt crystals were in close proximity to the fish, it is possible they would acquire a chemical burn prior to it dissolving. For the same reasons, I do not suggest adding the salt to the filter chamber either. This can and will harm the nitrifying bacteria, especially Nitrobacter. This particular bacterium is very sensitive to high salt concentrations. One other consideration in using salt, especially on very weak or ill fish, is not to over stress the fish by jumping right in with solutions higher then .1%. What I mean, is you should stage the additions in .1% increments, each separated by eight hour or so. As an example, if your goal was a .3% final solution, then add .1% now, another .1% in eight hours, and the final .1% eight hours after that. Do not forget to test the water after each addition, to verify you are dosing correctly. It is also important to use an accurate scale when weighing the salt.