You can do this much more elegantly with the VLOOKUP formula by making separate table mapping lower bounds to letters. The mapping table must be sorted by grade number ascending.

For example:

Table

A     B
0     D
49.5  C-
54    C
59.5  C+
...   ...

Formula:

=VLOOKUP(SomeCell, $A$1:$B$9, 2, TRUE)

Where $A$1:$B$9 is the range with the grade table. (The $ signs tell Excel not to move the reference if you copy the formula).
Passing TRUE as the last argument will cause Excel to do a binary search to find the value, which (as long as the data is sorted) is exactly what you want it to do.

Go to the Visual Basic Editor, and insert this code. I don t know what version of Excel you re using, but for versions before 2007, go to tools, Macros, Visual Basic Editor. For Version 2007 and newer , it is on the Development Tab which is not enabled by default.

Depending on how you want to link it, you could add a button to the page, or call it from the Worksheet_Calculate event.

This assumes that you have the student s total grade in cell A2, and will put the results in A2 and B2.

Sub Calculate
    dim LetterGrade as string
    dim Superlative as string
    Select Case Cells(1,2)
        Case  >= 89.500
            LetterGrade="A+"
            Superlative ="Pass with Distinction"

        Case  84.500 to 89.490 
            LetterGrade="A"
            Superlative ="Pass with Distinction"

        Case 79.500 to 84.490 
            LetterGrade="A-"
            Superlative ="Pass with Distinction"

        Case  74.500 to 79.490 
            LetterGrade="B+"
            Superlative ="Pass with Merit"

        Case 69.500 to 74.490
            LetterGrade="B"
            Superlative ="Pass with Merit"

        Case 64.500 to 69.490 
            LetterGrade="B-"
            Superlative ="Pass with Merit"

       case 59.500 to 64.490 
            LetterGrade="C+"
            Superlative ="Pass"

        Case 54.500 to 59.490
            LetterGrade="C"
            Superlative ="Pass"

        Case 49.500 to 54.490
            LetterGrade="C-"
            Superlative ="Pass"
        Case <=  49.490 
            LetterGrade="F"
            Superlative ="Specified Fail"

 End Select
        Cells(2, 1) = LetterGrade
        Cells(2, 2) = Superlative


End Sub

An easy solution would be to simply split the formula into two cells

=IF(O5>0.895,"A+",IF(O5>0.845,"A",IF(O5>0.795,"A-",<Other cell ref here>)))

Other cell:

=IF(O5>0.745,"B+",IF(O5>0.695,"B",IF(O5>0.645,"B-",IF(O5>0.595,"C+",IF(O5>0.545,"C","D")))))

While I prefer the Vlookup method (Slaks solution) for numeric parameters for the simplicity and elegance, an alternative which becomes valuable in case of non-numeric parameters is concatenated IFs.

=IF( Case1 , "Label 1", "" ) & 
 IF( Case2 , "Label 2", "" ) & 
 IF( Case3 , "Label 3", "" ) & 

..... and so on

For example, in this case:

= IF( O5 >= 89.5 , "A+" , "" ) & 
  If( AND ( O5 < 89.5 , O5 >= 84.5 ) , "A" , "" ) & 
  If( AND ( O5 < 84.5 , O5 >= 79.5 ) , "B+" , "" ) &

..... and so on for other levels.

The VLOOKUP solution seems the best. To add a VBA script, you could bring up the Visual Basic Toolbar, add a control like a Button and then double-click it. The code for that event opens in the Excel VBA environment. There you could add the code in VB, perhaps something like this.

Private Sub CommandButton1_Click()
  Cells(1, 2) = getValue(Cells(1, 1))
End Sub


Private Function getValue(ByVal argMarks As Double)
  If argMarks > 89.5 And argMarks <= 100 Then
    getValue = "A+"
  If argMarks > 84.5 And argMarks <= 89.49 Then
    getValue = "A"

  .
  .
  and so on...

End Function