When the winter cover's on, I don't test. Period.

I take the cover off, first, connect and fire up the pump and filter, and get it circulating. THEN I'll do my first testing.

Before I close I add 1 quart of Polyquat, wait 48 hours, then run the FC up to 15 and close. Then I leave it until spring unless I have to syphon off some water to prevent overflowing.