IMO, I'd say if you're within two standard error of the max score, then you're at or pushing the ceiling.

By that standard one could pretty safely say 22+ on subject area scores and 23+ on composite would be in the ceiling range.

I wouldn't say there is plenty of room before the cap above a composite of 20, considering that the next score would likely be at the ceiling.

...pronounced like the long vowel and first letter of the alphabet...