Do you start ovulation time window from first positive opk or the darkest?

Mr

So I believe I got a pos opk yesterday: they were the same color as the control line, took the tests around 4 pm.

However, I tested again today and got this: which are definitely darker than the control line.

So would I start the 24-36 hour window from the one I got yesterday? Or from the one I got today?

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