I'm not completely sure but my experience is:
-If the trained skill is 0-5.99 points - +1.20-1.50 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 6.00-11.99 points - +1.00 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 12.00-14.99 points - +0.80 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 15.00-17.99 points - +0.50 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 18.00-20 points - +0.33 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
For goalkeepers is different and I have low experience with developing young GK, but as far I remember it should be something like:
-If the trained skill is x-12.99 points - +0.50-0.80 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 13.00-16.99 points - +0.33 skill points improving for 1 value jump (CA 0.05-0.1, depending of CA)
-If the trained skill is 17.00-20 points - should be +0.15/0.20 skill points(I'm not sure) improving for 1 value jump (CA 0.05-0.1, depending of CA)
That's ONLY for goalkeeping skills (Handling, Reflex, One On Ones, Kicking, Aerial ability, Rushing out). When GK train non-goalkeeping skills improving is as above for field players.