Here's some light reading:
http://www.skyandtelescope.com/howto...tml?page=4&c=y
http://www.garyseronik.com/?q=node/8
Note depending on your sec. holder you may lose some width around the edge of your diagonal. Err on the larger side as the impact on contrast is minimal.
Old school says that a 0.5 degree illuminated field is ample to cover the full lunar disk. This equates to approx 11mm with a telescope of focal length = 1270mm (11= 0.5x1270/57.3). If you are a variable star observer you would want a greater 100% fully illuminated field to ensure reliable magnitude comparisons unless you were to limit your observations to narrow field eyepieces.