Fill blank cells with 0 or alternative number

[Spellbound]

Hi all

I have a worksheet which is basically just for data entry; however some entries only require minimal data, leaving the cells in 3 columns blank.

I have just come across this code in another post (courtesy of Simon Lloyd) which is ideal for general purposes.

Sub Fill_Blanks()
Dim MyCell As Range
For Each MyCell In Range(Selection.Address)
If MyCell = vbNullString Then
MyCell.Value = 0
End If
Next
End Sub

Ideally I would like to customise this code to do the following:

1) Restrict the range selection for use with columns D / E / F on Sheet1 for example, so that the range selection stops at the last filled cell in column C.

2) It may be necessary to change the zero value in either D / E / F at a later date, so I would like the code to have individual values for each column, if that makes sense.

3) This is not essential but can the code be set to run on a row by row basis for that specific worksheet. In other words, if there is no entries in D / E / F of row 25, when you move down to row 26, the zero input is automatically filled in rather than invoking a macro at the end.

Please note my knowledge of VBA is virtually zero!

TIA ...spellbound