Impossible. Pixel is related to image quality and % is related to the area of the screen that the object will occupy, regardless of size. Are different things.
To convert Pixels to %, you need to know the exact width of the screen in px
, to then relate the size of the object to the total available space.
But if the subject is RESPONSIVENESS, it means that you intend to create something generic to run on various screen types. In this case, discard pixels at once and use measures that adapt to the screen size, such as '%', 'em' among others.
And for specific cases, where certain content formatting looks good in larger resolutions, but does not adapt to smaller resolutions, use this feature: http://www.w3schools.com/cssref/css3_pr_mediaquery.asp. In this case it would be the "3+ styles" you quoted, but in some cases it is necessary".
Welcome to Stack Overflow in English. I recommend a brief reading of the How to create a minimal, complete and verifiable example, because your question, in the present way, is too wide.
– Woss