UserForm Change Event

Untested but something like this:
1. Add a new Class module to your project, change its name to CTextboxHandler, then paste this code in:

Option Explicit

Private WithEvents m_tb As MSForms.Textbox
Private m_rngLink As Excel.Range
Public Property Set Textbox(tb As MSForms.Textbox)
    Set m_tb = tb
End Property
Public Property Get Textbox() As MSForms.Textbox
    Set Textbox = m_tb
End Property
Public Property Set CellLink(rng As Excel.Range)
    Set m_rngLink = rng
End Property
Private Sub m_tb_Change()
    m_rngLink.Value = m_tb.Value
End Sub

Now add this to your Userform code:
at the top of the code module before any routines:

Dim colTBs as Collection

Now in the Userform_Initialize code you need:

Private Sub Userform_Initialize()
Dim oHandler as CTextboxHandler
Dim x As Long

set colTBs = New Collection

For x = 1 To 15
   Set oHandler = New CTextboxHandler 
   set oHandler.Textbox = Me.Controls("txtPTno" & x)
   Set oHandler.CellLink = Sheets("Personal Tax").Range("C" & x).Offset(x - 2)
   colTBs.add oHandler
Next x
End Sub