Thanks Art! All good information. Provides a great deal of food for thought.

It does look like there is some guess work still involved in determining first-flush volume. In this case it would be the old axiom, "Better safe than sorry". Especially if you are going to consume it rather than use the water as "gray" water.
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“Always remember the 6 P’s”
(Prior Preparation Prevents [censored] Poor Performance)