=IF(X2>=85,0.559,IF(X2>=80,0.327,IF(X2>=75,0.255,-1)))
Explanation:
=IF(X2>=85, 'If the value is in the highest bracket
0.559, 'Use the appropriate number
IF(X2>=80, 'Otherwise, if the number is in the next highest bracket
0.327, 'Use the appropriate number
IF(X2>=75, 'Otherwise, if the number is in the next highest bracket
0.255, 'Use the appropriate number
-1 'Otherwise, we're not in any of the ranges (Error)
)
)
)