At first time, revise your content, page title, page description and others to verify they match your desired keyworkds.
Think, today the meta keywords it's useless. You need to have your keyword present on your content, that's the more important thing.
Also, if some of your keywords are present on page title, description, or in-page titles (h1, h2, h3...) that's better too. If the keyword are at the url, best too.
If your ranking it's too low, revise your page for some things: First, your page are W3C ok (You can use the W3C page check), and then, use some free tool to evaluate keywords from page. Also, think, if your page are lack of content that's bad (You need a minimal quantity of content for get a better ranking).
Also, do not forget to avoid the use of javascript / documents (Word, pdf), and flash, because they all are external, on some cases, google can get it, but at another cases not. It's better if your content (With your keywords) are present on the target page.
I cannot recommend you a percentage of presence of a keyword, i think that's a very questionable thing. I personally use about 1-2% of keyword presence on the pages to get some rankings, and a good content between this keywords (I try to use them on the content, without force their appears).