Run macro when formula results changes based on different input cell

Hi all together,

I'm pretty new to vba and my knowledge is less than basic. However i've been able to successfully create a macro which automatically runs when cell value changes (by manual input/enter). But I need to change something but get stuck.

What I need is either the macro runs when cell "city" changes based on a formula (other cells are input) or, that city is changed manually but in the line IE.navigate "http://://www.zillow.com/" & Range("city").Value it will complete the URL based on another cell.

Hopefully you understand my problem and have a solution. Thank you very much in advance,

David

Here is the macro:

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Row = Range("city").Row And _
    Target.Column = Range("city").Column Then
        Dim IE As New InternetExplorer
        IE.Visible = False
        IE.navigate "http://www.zillow.com/" & Range("city").Value
        Do
            DoEvents
        Loop Until IE.readyState = READYSTATE_COMPLETE
        Dim Doc As HTMLDocument
        Set Doc = IE.document
        Dim aa As String
        aa = Doc.getElementsByClassName("value")(0).innerText
        IE.Quit
        Dim ab As Variant
        ab = Split(aa, ".")
        Range("homevalue").Value = ab(0)
    End If
End Sub