I find that a lot of C&C first time Players and GMs see the primary and secondary attribute rolls as a hard rule rather than a guideline. Instead of a set rules think of it as a starting point for every roll.So here is a way to look at it a slightly different way.
A 18 difficulty for secondary attributes is just the base roll.
A 12 difficulty for a primary attributes is just a base roll.
Now comes the GMs evaluation of the situation. He can now adjust that number up or down as he feels its warranted. My personal range is between -4 and +4.
Very easy task, But a roll still matters. -4,
Easy task -3, Tough -2 through 0.
Hard task +1 to +3
Very hard task +4.
Or give a bonus to the roll based on difficulty of the action being attempted. Between -6 and +6.
Very easy task, But a roll still matters. +4,
Easy task +3, Tough +2 through 0.
Hard task -1 to -3
Very hard task -4.
I know its not specifically expressed that the difficulty should be adjusted by Gm fiat. It is based on the attribute rolls of classic D&D. Which was subject to GM fiat.
An optional rule I follow is increased attribute bonuses. From the first chapter of the CKG is optional rules for greater attribute bonuses. I personally like this because I think the attribute should make a big difference.
If you like 3E math use its attribute formula. attribute/2-5= attribute mod.
And my call on adding level to attribute rolls. I only allow level to be added if you are preforming an action that is related to your class. After all your experience in that class is what that level represents. Just my opinion on this one.