Hi! I think there isn't direct relation between rom usage and ram. You can write code which is going to use whole ram or only f.e. 3 bytes in ram. All depends on your programing technique, language you are using and your ideas, and methods. Yes - I agree with Bordyn. But Bordyn is talking about "normal" code, you can always use the whole ram area for you variable. But if you are going to use rom for string, patterns or code which doesn't need ram, you can use whole rom without ram using.
Your question is very difficult and there isn't simple answer, I think.

Greetings,
Jacek