It was shortly before before QL appeared, if I remember correctly.
Used a variety of techniques, including measuring case-head expansion (which I'd already started to distrust, but can provide approximate info if used correctly). But the major "formula" was the one I worked out by crunching numbers from various pressure-tested sources: Any increase or decrease in bore area results in about 1/4 as much potential increase/decrease in velocity at the SAME pressure, using the same bullet weights. Used this by comparing the .264 with the extremely common data for the 7mm Remington Magnum.
Also looked at what data was available with other powders, and between the two comparisons came up with approximately how much velocity the .264 should be capable of with various bullet weights. Had already used this technique for coming up with .300 WSM data for an article in HANDLOADER, before any pressure-tested data appeared--and when tested data appeared my powder charges only varied by at most a grain of powder, and more often within a half-grain.
This includes the presently published data for Ramshot Magnum in the .264.
