generally, and this is old school talk here, we tried to keep within 20 (20 above and 20 below)
the same was used for working melee skills as well.
is that the best numbers, not sure, but its worked in the past with little to no problems.
so if i where wanting to train at 70 you could go from 50 to 90.
now, that said, your gonna get your best gains by sticking to stuff right in around where you are for skill.
that also said, many times while powergaming, ive worked skills up to 60, then gotten some cu sidhe's and ringed up and trained them from 60 real all the way to 120.
so ring up to 90, train up to 90 real.
take ring off and train up real to 120.
so in that instance im 30 away from the barding diff of the cu, but it still works good.
i think when your OVER the skill, you get more successful uses of the skill which tend to help.