@RRMX01 If the data type of your [Slope (%)] column is Single line of text then your formula will fails as you are comparing text in your formula.
Solution 1: If you want to continue using single line of text for Slope %, use below formula for calculated column:
=IF(Value([Slope (%)]) > 90, "PASS", "FAIL")
Solution 2: Use Number data type for Slope (%) column & use below formula:
=IF([Slope (%)] > 90, "PASS", "FAIL")
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