Overwrite Last Used Cells In Columns Via UserForm

i have searched this and can't find exactly what i'm looking for. I am hoping some one can alter a piece of script i am using. See script below. Currently it adds a comment in a text box to a new row and i would like it to add it to the end ( cell 15) of the last row used, not a new row. I am a newbie to VBA and am self teaching so the obvious is not really obvious.

Private Sub cmdErrorButton_Click()
Dim iRow As Long
Dim ws As Worksheet
Set ws = Worksheets("Log Book Data")

'find first empty row in database
iRow = ws.Cells(Rows.Count, 1) _
.End(x1up).Offset(1, 0).Row

'copy the data to the database
ws.Cells(iRow, 15).Value = Me.txtError.Value
ws.Cells(iRow, 14).Value = Now()

End Sub