Proper line count when printing

this example is printing lines of text to a printer

VB Code:

Public Class PrintingExample
    Private printFont As Font
    Private streamToPrint As StreamReader
    Private Shared filePath As String
    
    Public Sub New()
        Printing()
    End Sub    
    
    ' The PrintPage event is raised for each page to be printed.
    Private Sub pd_PrintPage(sender As Object, ev As PrintPageEventArgs)
        Dim linesPerPage As Single = 0
        Dim yPos As Single = 0
        Dim count As Integer = 0
        Dim leftMargin As Single = ev.MarginBounds.Left
        Dim topMargin As Single = ev.MarginBounds.Top
        Dim line As String = Nothing
        
        ' Calculate the number of lines per page.
        linesPerPage = ev.MarginBounds.Height / printFont.GetHeight(ev.Graphics)
        
        ' Iterate over the file, printing each line.
        While count < linesPerPage
            line = streamToPrint.ReadLine()
            If line Is Nothing Then
                Exit While
            End If
            yPos = topMargin + count * printFont.GetHeight(ev.Graphics)
            ev.Graphics.DrawString(line, printFont, Brushes.Black, leftMargin, _
                yPos, New StringFormat())
            count += 1
        End While
        
        ' If more lines exist, print another page.
        If Not (line Is Nothing) Then
            ev.HasMorePages = True
        Else
            ev.HasMorePages = False
        End If
    End Sub
     
    ' Print the file.
    Public Sub Printing()
        Try
            streamToPrint = New StreamReader(filePath)
            Try
                printFont = New Font("Arial", 10)
                Dim pd As New PrintDocument()
                AddHandler pd.PrintPage, AddressOf pd_PrintPage
                ' Print the document.
                pd.Print()
            Finally
                streamToPrint.Close()
            End Try
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try
    End Sub 'Printing    
    
    ' This is the main entry point for the application.
    Public Shared Sub Main()
        Dim args() As String = System.Environment.GetCommandLineArgs()
        Dim sampleName As String = args(0)
        If args.Length <> 1 Then
            Console.WriteLine("Usage: " & sampleName & " <file path>")
            Return
        End If
        filePath = args(0)
    End Sub
End Class

Im curious, how can I determine the correct number of lines when I have different fonts on different lines? They ar enot all of the same size... An approximate value isn't safe I think....

kind regards
Henrik