Understanding how elevation change affects your shot is a big part of good course management and hopefully to lower scores.
Your definitely on the right track using Google Earth, it is a great source for evaluating a course or even a particular hole
To your question, there is NO formula that will provide very accurate information.
- It is almost impossible to determine accurate elevation change that is greater than a few yards. So adding a club for every 10 yards is still basically a guess.
- The yardage correction needed for an uphill shot is going to be different than the yardage correction for a downhill shot with the same distance and corresponding elevation change due to the flight characteristics of the golf ball.
- The yardage correction for a 100 yard shot with 10 yards of elevation change is very different than the yardage correction for a 200 yard shot with 10 yards of elevation change.
So “eyeballing” it or going by “feel” are two very common methods, they are still only educated guesses.
If you are serious about game improvement I suggest checking into some of the products that will provide this information for you, Bushnell, Leupold or Nikon Callaway all work very well, but at a pretty high price.
Here is an updated link to an inexpensive product that will provide the same information at a reasonable price Slopetecgolf. Even if you use it on a few holes per round it will help you start understanding how to play shots with elevation change a little better.