It appears that the best readings require a good pre use rinse and then doing the test twice... the first test rinses away the rinse water, the second test gives apparently very accurate readings. Seems like a very cheap answer to that big question.

I had a leak in my fresh water system, but I was able to isolate it easily as my bilge is normally dusty dry... the only time I had water in the bilge was when the fresh water system was left on for awhile. It was just a matter of checking all the hose clamps.