Found this, It's a discussion about exactly this with some seemingly knowledgable people trying to get to the bottom of the Staniforth correction factor for angle. I'm not sure I'm able to apply it but I'll give it a go.
http://www.ten-tenths.com/forum/showthread.php?t=126399
There is this as well, but to me this is as clear as mud. I assume this is from Staniforth's book.
http://enderw88.wordpress.com/automo...g-rate-theory/