As time permits, I'll post a more sophisticated formula. The one I posted starts to become inaccurate above 6000 meters. The last factor needs to have a Delta/Rate of Change factor in it to slow down the rate of increase, which usually requires the use of exponential function. I've purposely avoided that because I'm not sure most players would enjoy having to (or be able to) work the equation.
This one is relatively simple and still allows you to hit your target the majority of the time and have some fun...and fun is what it's all about.
