There are a few things to consider when testing aquarium and tap water.
Always rinse the vial at least 3 times before a test, and try to be exact with volume.
You are dealing in ppm (parts per million) so the margin for error is high.
and after each test rinse the vial another 3 times before doing another test, if there is even a smidgen of residual left on (even dried on) it can skew results.
If I had access to DI water that's what I would use to rinse with.
And when testing for pH, use normal range pH reagent first, if it's at the top of its range, then test with high range reagent.
i would also recommend doing at least 3 tests for each chemical species to get a good result.
You are not wasting reagents, many aquarists end up with expired reagents, so don't skimp.
I was a chemist at a large drinking water facility, and any deviation from standard protocol often ended up with dumping skewed results, and starting over.
I'll make sure everything is rinsed thoroughly and that the water amount is exact. I'll definitely do a few more tests just to be sure!