Oh, ok, I sort of skipped over the “NRT to SEA on an A321” bit 
I figure that’s an edge-case. The Improved Aircraft Performance formulas would likely solve this issue, but the current static formulas just don’t permit any additional fuel in “section 3” of the payload-range chart. And deviations from optimum speed always incur an increased fuel burn.
Let me think a little about how this issue could be resolved.