Hey, I’ll go math-y with you all day. 🙂
I don’t think there’s a formula for this, but what you’re saying makes sense. A novel-length work should have many conflicts (5, actually) and several subplots (3-5). It should be clear to the reader that one particular conflict is the “driving conflict” and that’s the one that has to be resolved in order for the reader to feel satisfied. Of the other conflicts, some will get solved, some will make progress, others will end the book just the way they began it.
If a subplot gets so much attention that it begins to overshadow that driving conflict, you’ve got problems. It’s like a backup singer pushing the headliner out of the way and taking center stage.
It’s probably more feel-y than math-y. Good subplots should feel as much like character development as they feel like introducing a totally new plot thread.
Typically, a subplot should be sprinkled in with the main plot, like a thread wrapped around a rope, usually (but not always) for the length of the story. I think we have to think of it more in terms of focus than in terms of size though.
I hope that helps!