Questions about calcium, nitrate, and chlorine testers

[Muffin87]

1. How important is the quality of ro/di water used with the calcium checker? Is ro/di water testing 0 with any TDS pen good enough, or does it need it need to be pharmaceutical grade purified water?
   
   
2. In the dilution step of the nitrate test, does the nitrate-nitrite-free artificial sea water need to be produced from the same salt mix as the aquarium water I'm testing?
   
   
3. Is the HI761 Ultra Low Range Total Chlorine Checker based on the DPD (N, N-diethyl-p-phenylenediamine) method? Can it be used to test reef tank water, or only freshwater?
   I need a DPD total chlorine test to check my tank water for residual oxidants such as bromite, bromate and hypobromous acid, which need to be lower than 0.02 mg/L.

Thanks a lot for this!
Sorry this is full of technicalities, I tried emailing your aquarium specialist about the second question, as it was suggested in another thread, but I didn't get an answer.