The short answer, it's just a bunch of grinding through math for the purpose of limiting losses created by an 8829 for mortgage interest (that may be disallowed if over the limit) and real estate taxes (that may be disallowed if over the SALT cap).  I generally don't have Sch Cs w/losses so it's been mostly irrelevant in which section of the 8829 we deduct the interest and RE taxes.

My guess is that ProSeries and Lacerte haven't implemented this because you can run into those "circular calculation" problems where you bounce back and forth between standard and itemized, which then changes the way the 8829 is calculated which then changes whether you should itemize, ad infinitum.

I love that Congress keeps coming up with new and fun ways to put unsolvable math problems into tax preparation.

Rick