Why don't you keep your last entered sequence value in a Parameter table ?

(PseudoCode)
VB Code:
  1. NextIdvalue = GetNextSequenceValue()
  2.  
  3. INSERT INTO MyTable (Id, ClientName, CustomerNumber) VALUES (txtNextId.Text, TxtCustomerName.Text,[B] CustomerNumber & " /S" & NextIdvalue [/B] )
  4.  
  5. Function GetNextSequenceValue() As String
  6.     ' Look at param table ans return the new Sequence value + 1;
  7. End Function

Is this suggestion is helping ??