Well, I screwed this month up. Should have called on the day with the darkest line. Actually, should have called the day before, the first day I could definitively say there was a 'change'. If it turned darker the next day I would then postpone my appointment by a day.
Ugh. So frustrating.
When I called on Monday to ask what to do (my second line much lighter at that point) the nurse offered to have me come in anyway that afternoon...I asked her why and she said that yeah, there wasn't really much point as I would almost certainly be too late. I see no reason to waste the money. So I wait for next month.
Next month: temping, fertility monitor with first morning urine and 11:00 First Response sticks. All the gear, all at once.