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.