Three different ways to look at how Scott is comparing to Jennifer Pharr Davis's record run:
(1) JPD went 1603 miles in her first 35 days, to Scott's 1732. That would seem to put Scott almost 3 days ahead. However, Scott is hiking north while JPD hiked south. This means that Scott has the hardest part of the trail coming up, while JPD had a relatively easy section remaining. Scott can lose about 12 miles per day to JPD's finish and still get the record.
(2) 'Linear' pace. If JPD had hiked the same distance every day, after 35 days she would have been at 1649 miles. That's 83 miles behind where Scott is now. So Scott will get the record if he can maintain 40 miles per day over the last 450 (very steep and technical) miles.
(3) 'Scaled' pace. This compares how fast JPD covered each section to how fast Scott covered that section (and adds a bit to account for the fact that Scott has to hike 8 miles further, since the trail has changed slightly). It is still not quite a direct comparison because we're comparing days from the beginning of Scott's hike to days from the end of JPD's hike. By this measure, Scott has been behind for most of the hike after his early knee troubles, but has recently surged to a 7 mile lead. He is entering a section where JPD struggled, and if he can handle the terrain has a good chance to start pulling ahead.
For more detail see the awesome spreadsheet here -
https://www.dropbox.com/s/vh92xpsn7331f4h/Jurek.xlsx?dl=0