So this is some kind of user interface where users can set the font size they want for desktop, and you want to automatically modify the values they enter for mobile / small screen. Have I understood that correctly?
(???) To allow the user to optimize his experience on your site? The user can make any of these changes in his browser, if he wishes. Besides, in a well designed web page, the size of the font has been optimized by the coder to display at all screen widths. If you are trying to make up for a lack of knowledge of how to use breakpoints or scaling techniques, don’t; your ambition defeats the essence of responsiveness which is to adapt/adjust to all screen sizes and densitites without user intervention. Continue reading, practicing, experimenting and learning. Typography on the web isn’t a perfect world, yet; but it’s very good and gradually getting better. Read… learn about the obstacles and achievements. Sounds like you would enjoy it.
Here are a couple of current articles worth reading:
Viewport Based Typography
Options Panel are the parts of modern-day CMS so that a layman can make necessary changes. There are thousands of google fonts and various versions such as px, em etc and we do not know which google fonts and which dimension will be chosen by the client who is making customization through options panel in the admin. as such we have only one option left and that iss to handle the fonts sizes in media queries on the %age basis. even if we say fix some em value then one should be cautious that different fonts will have different dimensions based on the em value.