In theory, your global average would be if you could completely halt improvement (as well as not getting worse) at your current point in your cubing progression, solve an infinite number of cubes, and take the average of all of them. Of course, times fluctuate from day to day, you never know if you're improving past a wall or not, or if its just a lucky day, etc. So global average is just taking a guess at what that first figure would be.
A decent way is to figure this to graph out your best average of 1/5/12/100/1000, etc, and find a relatively accurate asymptote. Or you can just eyeball it. It usually won't be too much slower than your best average of 100