Let's start with the citric acid . citric acid is available almost everywhere so if you have questions about what you have just make a new batch. Once you have a known concentration you can then activate some sodium chlorite and measure its pH. Comparing the pH of the activated solution mixed from the unknown citric acid with that of the known citric acid will give you a "reasonable" idea if your original citric acid is about the same concentration as your known concentration of citric acid.
MMS is supposed to be a 28% by weight concentration of 80% pure sodium chlorite. On a Brix meter it will should read 28. Yours is reading 18.5 indicating that it has degraded some. Understand that the Brix reading is approximate.
This is not a problem because with sodium chlorite you can just adjust the amounts used and come up with the same results. Actually this is not the case with Miracle-Mineral-Supplement because it is measured in non standard measurements. Drops are inconsistent and vary depending upon dropper size and the technique used to form the drop. The drop size is also dependent upon the specific gravity of the solution. If standard measurements were used you would simply use some multiplier of the amount indicated to come up with the amount to use. With drops it gets more difficult because you now have to understand how many drop the original is based on and how many drops you are making for the same measurement.
You can also back calculate based upon available chlorine dioxide. The original concentration is 28% by weight of 80% pure sodium chlorite yielding 22.4% sodium chlorite. This gives you 224000 PPM available chlorine dioxide. Your mixture based upon the Brix reading is 18.5% by weight and we assume 80% pure yielding 14.8% sodium chlorite. This gives you 148000 PPM available chlorine dioxide.
1 milliliter of your 14.8% concentration would give you 5 PPM available chlorine dioxide if you added it to 29 liters of water.