@Mark ********
just so you're clear while you might have done that, Most offices have an 'apply for an extension when you have 30 days or less left on your current stamp, some (Bangkok, Chiang Mai, Buriram, Phuket, and a few others) have when you have 45 days or less, and a few (Jomtien) will send you away and say come back when you have a couple days left
Telling someone to wait until they have 30 days or less left on their current stamp is just a way of making sure that MOST people can get the extension issued when they go