How do I calculate the amount of preservatives one would need for a cleaning solution?
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).