How should purging parameter limits be established?

If you do not have any regulatory guidance to set parameters by then use something in the range of 3-5% of the reading values, or in the case of parameters where percentages will be too extremes, such as pH or DO, you can use a low range of .2 mg per meter which is about what the meter can actually measure. But the literature on indicator parameter stabilization will have a number of regulatory guidelines.