Hi John: I've been in the roofing business for 13 years. Whether we're doing a roof replacement or cleaning our guys measure every job from the ground. We then use a pitch multiplier and a waste factor (replacement only) depending on the style of home. The key to this working is first and foremost to get the pitch correct. I recommend a $10 protractor/angle finder that you can buy from any Ace Hardware or Sears. Place the protractor along the bottom edge of the rake board or on the roof itself. You can also place it against the underside of the deck in the attic whichever is easiest to get to. This will give you a precise measure every time. For technology geeks, you can download an app called Pitch Gauge 2.1 for your iPhone/iPad (maybe Droid?) that uses the camera to calculate the roof pitch. I would recommend field testing it to get the hang of it before relying on it for pricing. For an added cushion, I eliminate odd pitches since they are rare in my area and are usually the result of a mistake. The next most important measure are the actual dimensions. On the rake end, make sure you measure from roof tip to roof tip and not the foundation. Otherwise you will come up short. If it is a hip roof, add another 10% AFTER applying the pitch multiplier. Below are the multipliers we use. I hope this helps.

2/12 1.01
4/12 1.05
6/12 1.12
8/12 1.20
10/12 1.30
12/12 1.41
14/12 1.54
16/12 1.67

Try using the ad Pitch Gage on Android.

AC