There is no exact percantage. Just make sure it is prominent on the page in it's "exact phrase" yet still reads well... and if possible make sure there are at least a couple of variations + related keywords.
Do that and you will be fine.
It doesn't need to be "perfect" if you have good links coming in.
As far as exact percentage - 1% is a general guideline for the entire HTML page - but like I said there is no exact percentage and you should not worry about this.
You can look up your page using the AUKDAT tool if you want http://www.articleunderground.com/webma ... -tool.html