Thx for the fast reply, on Apr 14 nearing midnight west coast, no less, one day to go. Yeah changing it to zero was obviously just an edge case to see how the thing was working. I forgot about how (of course) the MFS would split-allocate principal balance and therefore the interest so that mostly explains. Though, now here's what's weird, it seems to be "giving" me about 60% (not 50%) of the mortgage interest. ($16K of $26.7K) when I look at the interest on Sched A vs the interest worksheet. (Sounds like I'll be buying audit defense.)
