Scroll Week Day 2: Attribute Modifiers

I'm not sure why attribute modifiers aren't taken into account in the DMG regarding scroll inscription. Honestly, I can't even take a cap off a fountain pen without getting ink all over my index finger. I can't imagine having to spend days (or weeks if you use the BX time requirements) trying "not to mess up" a scroll. The real question is how much or how little modifier there should be?

On the chart at left, my thinking is this... after a certain point toward the "high end" of the attributes (in this case, around 15), the chance of failure "flattens out" if you're "following directions" to create the scroll. But below that point, I think it would get proportionately easier to screw up. No offense to those magic-users out there with the absolute minimum attributes required for the class, but a magic-user with INT and DEX of 9 each should face a much higher failure rate for successful scroll creation than a magic-user with INT and DEX of 18 each, regardless of the level of the spellcaster or the spell. BTW, modifiers are cumulative, so yes... a spellcaster with a DEX and INT of 9 each would modify their chance of success by -150%!)