I can't find any way to enlarge that picture, so I can't see specifics.
Software, for the most part, is running the chip against an algorithm of what similar chips have achieved - nothing specifically tailored to your own.
Manually overclocking (in the BIOS) would be highly recommended - not only could you probably achieve higher, at a minimum you should achieve the same 4.4GHz software achieved, at a lower voltage.
As for specifics for X99.... I'd have to do some digging.
At a very high level, some of the this video should cover it... although there will be more specific, relevant resources out there.
View: https://youtu.be/Nj4ixZE7EWo?t=460
For a 5820K, 4.4GHz is probably just about the sweet spot..... and should be achievable with 1.25-1.3V.
Some people have pushed 4.7, 4.8 etc on those chips, but not sure what kind of voltages.... I would assume >1.35V.