First off under document setup, make sure the "compose to" printer is set to the printer that you are using, and the document output resolution is set correctly. Next under the print dialog box, make sure you are using the correct ppd for your printer, and under the Paper option make sure the scale is set to 100%
PM6 was never the best version of PM.
But if you set the page correctly, e.g. A4 297mm × 210mm, the margin of error is very small. Try some tests, like drawing a 100mm line in PM and measuring it on the printed output.
If you are using a basic domestic printer, it won't be 100% accurate anyway.