I believe the boost grows gradually and from 4 points down it kicks in for sure, but it seems to differ for each skill how big the boost is. For example, statistics have shown that if you train handling while having high OD, it gets a big boost, but the other way around is less of a boost.
As far as I know, most of the elastic effects are between 2 skills (except for rebounding, which is pulled up/down by IS and ID), so the trained skill doesn't have to be behind all other primary skills.
Here are the elastic skills according to Rhyminsimon's training simulator: (trained - elastic skill)
JS - DR
JR - None
OD- HA
HA - OD
DR - HA
PA - HA
IS - ID
ID - IS
RB - IS & ID
SB - None