How do I calculate the amount of preservatives one would need for a cleaning solution? I plan on making a very simple cleaning solution to clean metal contacts on PCB boards. It cannot contain anything that is flammable or hazardous so I've come up with the solution below.

Distilled Water ~92%
Ethoxylated Alcohol (CAS 68439‐46‐3) ~5% &quot;surfactant&quot;
Tetrasodium Glutamate Diacetate (CAS 51981-21-6) ~1% &quot;chelating agent&quot;

I am not making this solution in a perfectly sterile environment so I would like to add a preservative to increase shelf life. I've been researching for weeks and I cannot find anything definitive. I just want something that will easily dissolve into the solution above and not be harmful to metal. The solution will be applied to the metal via a cotton swab. Even if I find a suitable preservative how would I go about calculating how much to use?
My ideas for preservatives:
I think Citric Acid (CAS 77-92-9) would be okay. I think a combination of Methylchloroisothiazolinone (CAS 26172-55-4) and Methylisothiazolinone (CAS 2682-20-4) would be okay as well.
Other concerns are: Do I need an emulsifier? Is there an additional non-hazardous chemical I can add to decrease drying times?
Most producers/suppliers have recommendations for concentration ranges of their products. If you can´t get it directly, it is often possible to find the information on their websites (e.g. Schülke or BASF).
The ranges are typically around 0,1% - 0,5%, but different products are often mixtures of different preservatives and with different concentrations, so it can be difficult to compare products directly.
Citric acid has some biocidal effect but is not very efficient, so I would not recommend it.
Why do you need a chealant? You use demineralized water and a nonionic surfactant, so ions in the product should not be a big issue.
The CAS-number you refer for the surfactant covers a variety of chemicals, with different C-chains and degrees of ethoxylation. They are generally classified as mostly soluble in water, but it may be necessary to add a hydrotrope (e.g. Cumenesulfonate) or co-solubilizer (e.g. Butylglycol).