The "chi factor" was first mentioned in the "Introduction to Navigation: Star Fleet Command" booklet included with STAR TREK MAPS (Bantam, 1980).
Thi approach is by no means perfect. Instead of Cochrane's Variable (symbolized by the Greek letter "chi", which looks like an italicized "x") fluctuating due to the presence of gravity/mass as we know it, maybe it has something to do with the effects of dark matter and/or dark energy on warp drive.
This is one reason I felt that Archer's NX-01 ship was too fast for being 100 years before TOS. If his ship had been only capable of Warp factor 3 or 4, Cochrane's Variable would've made it possible to travel between stars without it taking years. With Cochrane's Formula, even early prototypes like Cochrane's Phoenix and the Valiant would've had at least minimal interstellar capability. This would explain how he was able to make it to deep space in TOS "Metamorphosis".