I've used this method before, it's best to do a yeast starter at about a quart of heavy wort with about 3 bottles of beer (just the yeast layer). Some people advise against all sorts of things, but you can get a good yeast culture using your method. If you don't have at least three bottles of good beer, start with a small amount of wort, maybe a cup. Then bump up til you have a roughly a quart.