Reportlab text centering with charspace

from reportlab.pdfbase.pdfmetrics import stringWidth
from reportlab.pdfgen import canvas
from reportlab.lib.units import mm


c = canvas.Canvas('test.pdf', (195*mm, 50*mm))
textobj = c.beginText()


def width(string, font, size, charspace):
    width = stringWidth(string, font, size)
    width += (len(string) - 1) * charspace
    return width


textobj.setFont('Times-Roman', 12)
textobj.setTextOrigin((100*mm - width("Some string", 'Times-Roman', 12, 3)) / 2, 5*mm)
textobj.setCharSpace(3)
textobj.textLine("Some string")
c.drawText(textobj)
c.rect(0*mm, 5*mm, 100*mm, 10*mm)


textobj.setFont('Times-Roman', 15)
textobj.setTextOrigin((100*mm - width("Some longer string", 'Times-Roman', 15, 7)) / 2, 25*mm)
textobj.setCharSpace(7)
textobj.textLine("Some longer string")
c.drawText(textobj)
c.rect(0*mm, 25*mm, 100*mm, 10*mm)


c.showPage()
c.save()