I put the parathesis on the wrong one is all. Try this:
VB Code:
  1. iX = 560 + ((200 - CInt(invSize.Width)) / 2)
  2. e.Graphics.DrawString("Invoice Number", bFont, Brushes.Black, iX, 250)

The fact that is does the division first is what messed it up.

Here is how I tested:
VB Code:
  1. Dim txt As String = "Center Me Please"
  2.         Dim defFont As New Font("Tahoma", 14)
  3.         Dim boxStartx As Integer = 560
  4.         Dim boxStarty As Integer = 275
  5.         Dim boxWidth As Integer = 200
  6.  
  7.         Dim txtSize As SizeF = e.Graphics.MeasureString(txt, defFont)
  8.         Dim start As Integer = boxStartx + ((boxWidth - Convert.ToInt32(txtSize.Width)) / 2)
  9.         e.Graphics.DrawString(txt, defFont, Brushes.Black, start, boxStarty)
  10.         e.Graphics.DrawRectangle(New Pen(Color.Black), boxStartx, boxStarty, 200, 30)