What are the rules for extending a 60-day tourist visa in Thailand, and which city is better for the process, Chiang Mai or Pattaya?

Morning all and thanks for the great info here. I'm hoping you can help with a couple of quick questions. I'll be needing a 30-day extension to my 60-day tourist visa.

1. I've read it can be done 45 days before expiry of the 60-day. But I've also seen only within 30 days of expiry. Which is true?

2. Also, I can choose between Chiang Mai or Pattaya to get this done. Which would be easiest, most efficient, less of a PITA? And any days to avoid in either office?

Thanks in advance for your help. :)