Hi Kafee31ju. This is a very common mistake. Purified water TOC limit of 500ppb cannot be used to replace the calculated limit. If your calculation established 30ppb you should keep it as your limit. Because TOC is a nonspecif test method one will have to assume that all TOC picked up during the test corresponds to the contaminant/analyte of interest.
By now you are probably wondering how to make a sample of PW pass a 30ppb limit. The TOC of the sample collected contains TOC contributed by the PW itself and the analyte residue. Test a sample of PW collected from same source of your residual sample and use it as a blank or deduct (calculations) from the TOC of the residual sample. For instance, if your sample (as is) TOC is 400ppb (arbitrary number) and the PW sample collected from the same source is 380ppb then your sample has a 20ppb contributed by the analyte of interest.
On last consideration, if you are picking a lot of noise (sometimes 25ppb, sometime 35ppb) and think that is the natural variation of the instrument or sensitive of the test method then you may want to consider an specific test method.