Chairgun can give you some of the answers, but it's a bit painful to do all the individual calculations, so I've tasked the ballistics engine of the windage game to generate a table for you to illustrate what Simon means.
So to go with the example Dean gave of a 45 yard target at an angle of 45 degrees you would need to aim 51mm lower than you normally would.
Its very common for people to drastically overestimate the angle. I haven't bothered to include the numbers below 15 yards or 15 degree's because the difference is negligible.
With regards to the Riflemans rule - it's a reasonable approximation beyond about 30 yards, but below that you should ignore it, because it's wrong - sometimes by a huge margin!.
(Download as a PDF)