Accuracy of Patek Philippe Watches
I contacted Patek Philippe regarding the expected accuracy of their mechanical watches and they were very kind to reply very promptly (within 24 hours).
With regards to your question regarding timing, our current range for a mechanical watch would normally be between -2 and + 3 seconds a day.That places them at 'twice' as good as the ubiquitous and actually quite tough COSC standard of -4 to +6 s/d.
Please do not hesitate to contact me should you have any further questions.
With kind regards