Your ability with a spell is represented textually by Crawl,
but the relative rankings at the bottom end are not very
intuitive. Here are the rankings in order:
The 'spell failure' number is compared to 3d100/3 in order
to determine whether the spell attempt fails or not. Thus a
'Poor/Fair' spell will succeed roughly 50% of the time, an
'Excellent' spell will succeed 99.9% of the time and a
'Terrible' spell will almost always fail.