In reply to Ands:
It's a bit of a black art, and if you get it wrong, someone will complain!
Best advice is to spread yourself about as much as you can in order to get a feel for grades in different areas (e.g. Northumberland, Yorkshire, the Peak, Font, the States etc) as well as closer to home.
Most important thing if you're developing somewhere new is that you get the grades of the problems right relative to one another. If you need to adjust everything up or down later, it's less of a big deal. It's also worth getting someone else to test-drive your problems and grades before going public.