Cross-chaining in terms of angles
Rear cogs on a 9-speed are about 4.5mm apart, center to center.  A typical chainstay length is a hair over 400mm.  The arctangent of 4.5/400 is about 0.65 degrees.  At such a shallow angle you can simply multiply 0.65 times the number of cogs you're spanning to come up with the angle that is spanned when you shift that number of cogs.  In other words, shifting from cog 1 to cog 9 would span 8 cogs and produce about a 0.65 x 8 = 5.2 degree change in chain angle.

Front cogs are probably a little bit farther apart, but not much.