It is a very complex thing since sometimes even if you dilute 10 times, the COD values can follow a pattern that is not linear depending on the spiked solution you are using.In case of dilution of the sample the dilution factor must be known. The dilution factor is the total volume of the solution divided by the stock volume but some of the experience dilution of samples to be tested for COD does not produce accurate reproducible results it may be error of 15-30% depends upon dilution. You might have to make chemicals as per the standard methods rather than using kits. Moreover there are certain dyes which may interfere with the COD measurement and give wrong values. You might use TOC values as the indicators rather than COD values.