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).

Any javascript effor is invalid for SEO. Google doesn't read them.

You can try to make it on PHP, it's not complex, search your an replace with meta desc=$var and <title>$var2 (and, also, </head>). Then you can set the meta desc and page title with a variable in your code, and then this effor effectively have SEO value, because when the search engine fetch the page they have title and desc.</p> <p>Maybe this is more work than JS form, but also it's better for SEO and web itself (The JS run takes time on client side).</p></title>

No, that's not true.

Think SASS or SCSS are processed at server-side, and get a normal CSS stylesheet to the client.

SASS or SCSS can help you to manage your CSS files and classes, but you need to pre-process them to generate the final CSS sended to the client. If the final product doesn't change, seo will not change too.