I don't know that there's any ratio quoted; you charge your unit with the amount specified by the instructions, then add more if it doesn't seem to be doing the job. My C-Head takes 1 1/2 to 2 one-quart jars of peat moss; the others are larger and take more.
Of the three we've been talking about here (Air Head, Nature's Head and C-Head), the C-Head is definitely the smallest and lightest. That doesn't mean it's flimsy, though.
There are basically three models, if I remember right. The original, which I have, is great for setting against a vertical bulkhead. There's another one with a smaller footprint, that fits into corners better. And the third one has the bottom clipped at an angle in the back, so it'll fit better against the sloped side of a boat.
I'd post links or pictures, but I haven't reached the magic number of ten posts yet....
I think I'm allowed to post pic's now... let's give it a try.
The problem with my installation was that there's a plumbing and electrical chase against the back wall that holds the toilet out about four inches (thanks, dad-in-law who never saw a job you couldn't mickey-mouse....). The space was so small to begin with that even with the old RV flushing toilet I had to open the door to wipe my bottom without leaving a forehead print on said door. With the C-Head being four inches closer to the door than the old toilet that flared out over the chase, I just leave it ajar now. No biggie; I'm the only one here.
The other problem with the chase holding the toilet off the back wall was that it left the toilet lid unsupported in the up position. I took care of that by installing a pocket shelf on the back wall, that's just low enough to catch the top of the toilet lid. End of problem - and I wound up with a handy place to stash toilet-related supplies, like the spray cleanser with vinegar I use for the urine catcher and the baking soda I mix into the peat moss.
It's also a great place to stow the crank and lid when I'm using the toilet.. No more just dropping them on the floor, or leaning them in the wash basin.....
And by the way, I shouldn't rag on my father-in-law; I was mostly joking. He originally picked this motor home up cheap somewhere and put an ungodly number of hours into bringing it back from the dead, intending to sell it for a profit. Then he decided I needed it because I work away from home - and just flat gave it to me. I've never added up what he's saved me in rent for the last 12 years by doing that, but it's gotta be a bundle.