Len, Jeff
All the Cosine calculation does is allow for ,in a very general way, the change in gravity acting perpendicular to the Line of Sight.
In Jeffs example, the time of flight is 23.2% longer than for a 500Yard target, but the force of gravity acting along the line of sight is 23% less than normal gravity. It works the same as the way the cosine rule is used to find the wind acting along the line of sighth. Eg, a 10 MPH acting at 1 O' Clock when the target is at 12 o clock, will have the same effect as a 5 MPH wind acting left to right.
5MPH=10MPH * Cosine(60)
Depending on accuracy required, it works very well at short to medium ranges, but just like everything else in long range shooting, small inaccuracies in calculations give large inaccuracies in targeting solutions at long range.
For long range, it is best to play with a ballistics program until you get some angled drop charts that match what you see in the real world.
Rgds
700