Atm most skills shows how much they will be increased when you try to enchant them. I never found an acurate table with all the skills enchanted with all options, was trying to build one but since ncsoft changed the enchant skill window i quited the task.
There are some general rules:
a) +1 usually gives a high bonus. OEing for +2 and up gives minor bonus.
b)There are some points which give higher increase. Usually these are +4, +8 or +10 etc.
c) Debuffs need higher oe to land on higher lvl players/mobs. There is a table for the necessary enchant lvls for each enemy lvl in l2 b lah site, search Diego Vargas posts if you can't find it otherwise. For example shield bash as a lvl 77 skill will land on 77 and lower lvl with 80% chance. To keep the chance 80% against 78 players/mobs you need to go to +1 or better +2, 79 lvl +3 or +4, 80 lvl +5 or +6, you get the point.