Find the probability (percentage) of staff rated as poor.
Take 1 - P(poor)
This gives you the sum percentages of staff with fair and good ratings.
Then, divide this by 2 to get the percentage of fair and good ratings.
So, now depending on what type of z table you have, you can proceed with two different ways.
Find the z value for P(X > z) = P(Good)
or Find the z value for P(X < z) = P(Poor) + P(Fair)
When you got the z value, convert it into X.