It's fairly easy to find out this ratio of advertising expense to sales in your line. Check trade magazines and association. Your particular situation may dictate that you want to advertise more than or less than your competition. Average may not be good enough for you. You may want to out-advertise your competitors and be willing to cut into short term profits to do so.
Growth takes investment.
No business owner should let any method bind him or her. It ensures that your advertising budget isn't way out of proportion for your business. It's a sound method for stable markets. But if you want to expand your market share, you'll probably need to use a larger percentage of sales than the industry average. Your base can be last year's sales or an average of a number of years in the immediate past.