Thumper to pot sizing question

[Warno]

I understand a thumper should be 1/4 to 1/3 the size of the pot but my question is, does that mean total volume or actual gallons? What I mean is if I have 30 gallons of liquid in my pot, do I need minimum 10 gallons, going with 1/3, of liquid in the thumper?

I haven't actually read a post that talks about actual fluid levels in the pot compared to thumper just the 1/4 to 1/3 rule of thumb. If it's already been answered I do apologize.