How to calculate Forecast accuracy

Hi,

I've been trying to get my head around how to calculate a forecast accuracy. Here is how I usually set up a simply formula in excel:

First I calculate the absolute variance between sales and forecast. I do this for each individual "Material" (product).

absolute variance = ABS(Actual sales - forecast)

Then I summerize the total ABS variance and the total forecast to get the Forecast Accuracy:

Forecast Accuracy = 1 - ( Total ABS variance / Total Forecast )

(at the top you can find a small sample)

If anyone has an idea it would be greatly appreciated. This is getting on my nerves :)