My method for testing with a handheld meter is :
I always use a nice clean, clear glass drinking water glass, washed by hand in very hot water so it has no soap scum, residue or dishwasher rinse aids on it.
I first draw a glass or tap water and insert the meter allowing the reading to stabilize. Record the reading and triple rinse the glass and meter in RO/DI or distilled water.
Next draw a glass of RO only water and again insert the meter allowing the reading to stabilize, record the reading and triple rinse the meter and glass with RO/DI or distilled.
Finally draw a sample or RO/DI, insert the meter, record the readings and do a final triple rinse. Put the meter away with the cap installed and place the glass upside down in a clean area so it is clean for next use.

Its amazing how much things like hard water spots or stains, dishsoap reside and dishwasher rinse aids can skew TDS readings, especially when you are at the low end of the meter range like RO only and RO/DI, even a bit of dust or salt will throw it off significantly.