How much coffee grounds do I use for a large percolator? For the 10th year in a row, I have charged myself with the duty of brewing coffee for parish's Fall Festival.  Usually, I am provided with a giant tub of Folgers and told I might want to rinse out.  They also give me a little card that says how many "perks" I'll get out of every cup.

This is the kind of coffee maker I'm talking about, you put the coffee in a metal hopper in the pod and water splooshes out the middle and burns you when you try to open it:



I've got 3 coffee makers: I can't remember the exact sizes, but I think they vary between 35 and 100 cups. I usually make a pot of regular in the big one, a pod of decaf in the medium and a backup pot of regular in the small one.  

I am going to purchase big bottles of filtered water for this endeavor and also to get fresh ground beans from a local roaster the night before. 



What I really want to know is:


What is the actual cup-to-coffee ratio.  I know 1 Tbsp to 6 OZ and could do the math (But that's like 8 Cups of coffee grounds for 100 cups of water which I believe is chest-hair territory)
Given the ideal ratio, how far can I deviate before I sink into bathwater territory?
I am a big fan of ignoring volumetric measures and using weight instead.

Especially for a huge pot like this one, any old kitchen scale should be precise enough to allow a good dosage (for just a cup or two, you need more precision). The general recommendation for filter coffee is 


  60g coffee per 1000g water 


(And remember that 1000g =1kg = 1l.)

As for the "bath water territory", I admit that I have gone down to "1 heaping teaspoon per mug", which translates to ca. 7g per 200-220g or 30-35g per 1000g. That's pretty close to what aficionados call "bath water", but I'm sometimes a whimp when it comes to filter coffee. 
If you are dealing with the average crowd of American (weak) coffee drinkers (sorry for bashing the cliche here), I think you can easily go down to 50g/l or even slightly below without the majority even noticing. 

But as you are working with a small local roaster, I suggest you get a small sample beforehand and run a little test. Note that his coffee might (should!) brew differently from the supermarket sawdust. You might want to choose the right roast if you're going to lower the ratio. Tlk to your roaster: Factors that should be kept in mind are the type of machine and the fact that the water will probably be very hot, so the selected roast should be able to withstand it somewhat.

Also, the grind will be important, but "medium" should be a good starting point. If your machine needs coarser grounds, you will have to increase the coffee by 5-10% to get the same result.

Finally, even if you did some test runs at home and think you perfected every parameter, that kind of machine you will be using is not designed to maximize the coffee quality, but the output - that's just how it is. The results should still be significantly better than "get a few cans of [you know which brands] and boil them to death with chlorinated water". Most of your guests will enjoy the fête and the community and pay only little attention to the coffee - they will taste your effort and appreciate your dedication nevertheless.

(And if you do all that to make sure you can have a decent cup of coffee at the festival, I still think that's worth it!)