The average results of using a certain skill depends only on your level, and
on the efficiency % of the skill (which you can check at guilds with the
So, if you are level 25, and you know a spell at 90%, your average casting
level for that spell is 90% of 25 = 22.5. On average, it will cause damage
as a level 22.5 spell, it will be hard to save against as a level 22.5 spell,
and so on, regardless of what other spells or skills you might know.
Levels above 25 count as 1/10 of "true" level: so, a level 40 legend has a
"true" level of 25 + 15/10 = 26.5; a level 40 legend who knows a spell at
105% casts it at an average level of (26.5 * 1.05) = 27.8.
The numbers calculated above are an average; the actual casting level can vary
according to factors like spellcasting effort (quick spells have lesser
effects, thorough spells have greater effects), zone alignment, and so on.