is it possible to use too much R-0870 DPD powder?

No but you can get inaccurate results from adding too little powder. A good rule of thumb is to add enough powder so that there's always a little bit of undissolved crystals on the bottom after that first initial swirl.

Real high Chlorine levels need a little more powder so that there's some undissolved crystals left on the bottom.
 
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