In my understanding of window of tolerance or rather, what Peter Levine refers to as "range of resilience,' is a less fluid/flexible concept. "Range of resilience" refers to the calibration of the body's stress response system. Heart rate variability is an excellent proxy for measuring one's window of tolerance or range of resilience. Otherwise stated, if someone has low heart rate variability then they have a more restricted range of resilience. Here's what Bessel Van Der Kolk says,
"Poor HRV (heart rate variability) - that is, lack of fluctuation in heart rate in response to breathing - not only has negative effects on thinking and feeling but also on how the body responds to stress.... Failure to keep this system in balance is one explanation why traumatized people are so vulnerable to overrespond to relatively minor stress. " p. 269, Body Keeps the Score
When I am working with folks around self-regulation, I am trying to offer them strategies and practices - that if engaged regularly and habitually - will have a potential impact on recalibrating their body's stress response system (which is what we are referring to above). We fail to really "settle ourselves" sufficiently in the way we conduct "business" in schools, health care settings, even mental health settings.
Anyway, this is my understanding of window of tolerance / range of resilience. Most of us need ongoing opportunities to widen that space...