Pkoop-I don't think either of us are going to have legit sources on this. But my statement comes from experience and from sitemails with several successful vets (pepwaves, jsajsa, killbatman, Jdno, and many more).
similarly, players had different thresholds for the number of scouting trips required to pull them down. you could pull players down with as few as three scouting trips and my personal record was 52, yet 10 seemed to be the magic number that worked on the vast majority of recruits.