Hi Magik, Welcome to the Forum!
For MMS with 80% sodium chlorite powder or flakes, the 28% is the percentage by weight of the sodium chlorite in the solution. So, you will need a scale, hopefully one that's accurate at least to +/- 1 gram. Then take the amount of water that fills your container (glass or plastic) a little over 3/4 full (or take any amount of water you want to make into MMS) and weigh it, then divide the weight by 72, and multiply that by 28, to get the weight of the sodium chlorite (NaClO
2) flakes to add to the water. Or you can just multiply the weight of the water by .389 (a good approximation) for the NaClO
2 needed.
If the temperature isn't too cold, you can just wait awhile and the sodium chlorite will dissolve completely. Or, you can put your container of water and NaClO
2 into a hot water bath (a pan on the stove, for instance) to speed it dissolving.
Use distilled water if you can, but any water will do. If your water has any pollution or pathogens in it, the sodium chlorite will clean it up
Hope that helps,
Bruce