In-game messages and their minimal ranges using percentages:< 00~00 %: There is no chance of success.
05~15 %: The probability of success seems very low.
20~35 %: The probability of success seems low.
40~55 %: The probability of success seems to be about fifty-fifty.
65~75 %: The probability of success seems high.
?~? %: The probability of success seems very high.
100~100 %: There is no chance of failure.

It would be plausible to assume that the categories are 0, 1~19, 20~39, 40~59, 60~79, 80~99 and 100, but it is hard to tell, especially since there have been reports of 'guaranteed' grinds failing; perhaps the two extreme categories cover 0~4 and 95~100% instead.

Guide written by Tycho.