I learned this because ProSeries does NOT optimize the time window for you so I always had to do it manually.  At least Lacerte takes a stab at it (which may create a false sense of security).

You need a window of time with at least 330 qualifying days out of the US.  Most years the best result means exactly 35 days in the US.  So if there were no trips in 2021 or early 2022 I would expect the optimal window to run until April 19th 2022, or 109 days in calendar year 2022 (-1 day if there's a date-line variance--i.e. left Singapore on 3/16 but arrived in US on 3/15!)

But if there were earlier trips to the US in that 365-day window, they essentially shorten the end of the window because they've already been "used" so you can't tack them onto the end.  My guess would be the same as Jensen's, there were a few days (9?) in the US earlier in the window causing it to only extend to 100 days in 2022.  But I just pretend to know what I'm talking about, @itonewbie is our resident expert. 🙂

Rick