Font scaling from desktop to mobile

I am a designer and an issue I run in to all the time is how to scale font sizes down for mobile, and how to communicate that to a developer without 'just eyeballing it'. For instance, in my style guide, can I instruct a developer to decrease a font size by, say, 30% on mobile (as an example)? Is there any sort of golden rule to follow? This seems like such a no-brainer but I can't find this information anywhere.

I'm also very confused as to pixels vs points vs em. We use XD in our studio, which uses point sizes. Those point sizes don't always translate directly in to the correct size in the dev environment. They often appear much larger.

Can anyone guide me to a great resource or advise on the above? I'm so confused!

TIA