The way I do it is I generally just count from my last meal. I have a changing weekly schedule so it’s hard to stay with the same eating period, but as long as I start counting from my last meal, I can make sure there is 16 hours between that and the next one. Sometimes that means your eating period may lengthen or shorten, but your fasted period is still 16 hours.
I dont think you’re supposed to make radical shifts in your window, but an hour or two here or there wont matter much. Personally, I never eat before 11am (meaning sometimes it’s 11:30, sometimes its noon, sometimes its right at 11am), and I jut stop whenever dinner is finished, whether its 6:15pm, 7pm, or whatever.