My only issue with the use of percentages is how do you get a 99% chance with a d20? You'd need to roll a 19.8 somehow to achieve that..
99% is rolling above 1 with advantage. The odds of getting a 1 with both dice is less than 1%, so 99% is the closest percentage that is a whole number.
EDIT: Technically it's 99.75%, so 100% would be closest, but that would be misleading because you can still miss.
