UDF returning #VALUE when another Excel file is opened

Hi. I'm new to the world of Excel programming, so I hope I'm not being too simple here. I've Googled around for the answer to this and couldn't find a simple fix.

I've created some UDFs for a specific Excel file I'm working on. Here's the code:

Function QuantityWithExtras(quantity)
    QuantityWithExtras = WorksheetFunction.RoundUp(quantity * (1 + ([extrapercentage] / 100)), 0)
    Application.Volatile
End Function

Function ExtraQuantity(quantity)
    ExtraQuantity = WorksheetFunction.RoundUp(quantity * [extrapercentage] / 100, 0)
    Application.Volatile
End Function

Function IfNum(quantity)
    If quantity = "" Then
        IfNum = 0
    Else
        IfNum = quantity
    End If
    Application.Volatile
End Function

Sub SelectAndSort()
'
' SelectAndSort Macro
' Selects the appropriate range and sorts Z-A based on the "Open" column. This removes blank lines, allowing the menu to be pasted into Epicor.
'
' Keyboard Shortcut: Ctrl+t
'
    Range("B7:M74").Select
    ActiveWorkbook.Worksheets("EPICOR ENTRY").Sort.SortFields.Clear
    ActiveWorkbook.Worksheets("EPICOR ENTRY").Sort.SortFields.Add Key:=Range("B7" _
        ), SortOn:=xlSortOnValues, Order:=xlDescending, DataOption:=xlSortNormal
    With ActiveWorkbook.Worksheets("EPICOR ENTRY").Sort
        .SetRange Range("B7:M74")
        .Header = xlGuess
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With
    Range("A7:M74").Select
End Sub

When I open my file, everything works fine, and the cells using those UDFs update as they should. When I open another Excel file, though, the cells using the ExtraQuantity function return #VALUE, and I have to double-click on the cells and press ENTER to get them to calculate correctly again. I can save at this point and reopen and everything's fine, but it errors again as soon as I open another file. What should I add to my functions to keep this from happening? Thanks.

JosephP, that seems to have fixed it! Any insight as to why using a named range was causing a problem? (I assume it's to do with the newly opened file not having that named range, but it seems odd that Excel would get hung up by that.)