OPK question

Tara
If your opk gradually gets darker than the control line over the course of a few days, is your positive the first day it's as dark as the line or is it the day it gets the darkest of all your tests? 
I know that a positive is as dark or darker than the control line.  
Just wondering if you get as dark one day, then darker the next.  Which day do you calculate O from? 

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