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I don't think many SEOs really work to a strict percentage - I think that would depend too much on the content that's on the page.
Instead its pretty much about minimising the amount of code that's on-page and putting it off-page where possible and practical - usually by using external CSS stylesheets and JS files instead of putting your CSS and JavaScript inline or using HTML styling.
You could also look at content positioning in your code - is there an effective way to put your page content higher in your HTML, above any code that you can't move out? Many sites use static positioning in CSS to acheive that - putting a DIV for the page content at the top of the HTML code and then positioning it with CSS.
Having said that, I think the SEO benefits of these techniques are minimal. Historically they may have made a bigger difference, but now they're less relevant.
Instead, moving content out into external files can help with server load and page load time, because once the user has visited a page once they don't have to reload the CSS / JS content for other pages.
Cheers,
Jamie