Calculating forecast accuracy as a percentage

I am wanting to calculate forecast accuracy compared to sales where I have one column with actual sales figures and two other columns with forecasts. What I need to do is display the accuracy of each forecast against actual sales in terms of a percentage.

Simply showing the percentage difference is not good enough (can be anywhere from -200% to +200% as our sales guys are rubbish at forecasting), I need to show the accuracy as a figure from 0% to 100%.

I've been mucking around with this for over three hours and not getting anywhere.

Can anyone provide some assistance?

Thanks for the response Ben. I was hoping to be able to go with a percentage accuracy figure but am happy to go with an efficiency figure instead as you suggest.

However, do I assume correctly that a figure closest to zero is the best when measuring the efficiency of the various forecasts against actual sales (-5 is better than 20)?