Page 162 has some discussion on 'when' sex is determined. The "I wonder" problem I see with this is that higher temps also influence a shorter incubation period than lower temps, so 'when' that critical time is also seems like it will shift with temps. I've never had the moment needed with the right person to tease this out..
The "when" is a period of time. And that, in "days", will vary depending upon incubation times. That's why I feel it better to state Stage 16 to Stage 22. That is the bracket of time in which to work. I believe the effects also are cumulative. So many degree-hours above a certain Temperature in that period are required for the proper hormonal environment to achieve the proper estrogen levels. So to me, and my experiments, I see the "when" not as a point in time, but a bracket of time in which I have to apply enough "degree hours" to effect the female outcome.