This is easy using Google Earth - you simply hover the mouse over the start and finish points, and look at the value given for "elevation" (elev):

Get the difference between the two using your calculator, and add it as an attribute of your hill.

Hill Bomb uses this value to calculate the decline of your hill. An example:

On a hill that's 200m long, with a 100m height difference, for every 2m that you travel forwards, you're going down by 1m. That's a 50% decline, i.e. you're dropping at a rate of 50% of distance travelled. The higher this number, the steeper the hill!