MMS is a 28% by weight solution made from 80% technical grade sodium chlorite powder. This means that it ends up at 22.4% sodium chlorite.
If you take 1 part of the 22.4% solution and add 3.48 parts of water, you will end up with 4.48 parts of 5% sodium chlorite.
For example, if you take 10 ml (2 teaspoons) of Miracle-Mineral-Supplement and add enough water to make 45 ml (9 teaspoons), you will end up with 45 ml of 4.98% sodium chlorite. While not exact, this should be close enough.