Can I apply for a re-entry permit immediately after getting a 30-day extension on my single entry tourist visa in Thailand?

Hi

I am about to get a 30 day extension on my single entry tourist visa, and want to immediately after I get it apply for a re-entry permit, so that I can exit and re-enter for the 90 days (visa expiry date) and so get another 60 days stamp (+30 extension...) = 6 months on 90 day tourist visa.

possible?

How does the re-entry stamp work? on the application form they ask for travel dates... is the permit limited to that?

online I see that the permit stamp has "valid until" and "length of stay until", what will determine those dates?