I was just wondering why there is no reference to it anywhere?
Secret weight control folks' business.
Generally, such stuff may make a sentence in weight control or loading manuals but, more generally, it is background calculation material which surfaces only as some sort of final guidance result.
My guess, in this case, is that the formula derives from someone's idle playing with the graphs over a beer as Matt implies.
You might note that the stab from the load sheet is not always the exactly the same as the actual one measured by the ECAM.
Generally, ANY manual trimsheet will be tweaked to suit the design error analyses - if it's all done by computer there still is a need for such analyses and workarounds as the assumptions built into the loading system contain approximations and fiddle factors which only go so far to achieving end accuracy ... the aim is to end up with the aircraft's departing inside the envelope rather than a strictly accurate MAC value from the sheet.
The latter goes into the stab trim setting lookup which, in turn, is generally a little in error so far as the AFM data might be concerned.
Mind you, this presumes that the designer is competent .. I have reverse engineered numerous trimsheets over the years to figure out why they don't do what they should .. only to find basic errors getting through the design process.
As a consequence, the envelope limits as printed will have "apparent errors" should the enquiring pilot compare TCDS data with the trimsheet and other calculation inputs will be tweaked to end up with something which keeps things safe and sound rather than output-numerically exact.
I haven't any A330 background but, should the OP like to post some relevant data, we can comment on the specifics rather than in generalities.