Well, he sure had a huge sense of humor and he liked to pull legs and make tongue-in-cheek jokes, but he also had a lot of respect for the fans who loved this sort of details because he loved that too. So yeah, he might have added the 2.5% as a bit of a joke, or not, but he was also quite maniacal when he came to stuff like this, loving to work with graphs and charts, and logical grids. He's that type of world builder. He had full lists of all the Amyrlins since the Breaking, with their Ajah, ages, deeds, the reasons why they were raised etc. 99% of this stuff never showed up in the series, but it's how he worked.

He had used those numbers for years, he had to know that the Tower would turn down 40% of all potential channellers, it gave him a basis to start with, but he was fully aware the sample among AS (not so among WF, WO) is so small it deviated from the stats (as it did with the new novices). He used all those numbers to figure out how many Wise Ones and Winfinders he should have in relation to their population, made charts of their strengths etc. He loved to have that kind of information on file to be able to just turn to his notes if he wanted a character to pass comments in the vein of "she was stronger than more than half the Windfinders". Don't forget, RJ's notes are considerably larger than the whole published series, and he used all sort of special purpose work files. He had this document with the AS characters arranged by level, as women on different levels had relationships of deference, increasing with the gaps. The Sheriam entry for e.g. would also list her exact strength, her time as novice/accepted, specific comments she had made about her abilities, Talents she might have and if he had used that or not, and it also gave the level of deference for women she interacted most often with. RJ just had to look at that and in a scene with Sheriam's circle he knew at a glance how to approach a conversation between them, if it should be Myrelle or Beowyn who interrupts etc. Then if he had to bring in a minor character, he'd just looked at her entry to see her level, and knew how she should act, and if wanted a sister to act specifically he could double-check and see if anything contradicted that, and if not he could change her strength. Etc.

His character files were in prose and like more or less detailed biographies, with motivations and so on. More the sort of stuff he'd look at before starting a new book than something to refer to as he wrote.

Another type of document he used were extremely detailed and complex charts (Brandon said even Maria has problems navigating them as RJ had all sort of color codes for everything... what happened on screen, what happened off-screen, if it could be altered or not, stuff that might happen, projections of main events for "global outline" of the series and so on and so forth. Apparently it's only RJ who was comfortable with that chronology file (Brandon wasn't, he relied on Maria who herself often preferred to rely on Tellings of the Wheel if the information was one that was confirmed by the text, she looked into the chronology of RJ when ToW was in doubt).