That's a big pressure cooker! Mine is a 12 quart, I believe, but it only takes a few inches of water in the bottom to run it for a couple hours. Mine doesn't have a rack in the bottom, so to separate my jars from the bottom I use some jar lids and aluminum foil. With that I usually have only a little water touching the jars. I don't measure it out, I just put in a few inches worth of water, but now there is a line on the side from using it, so I just fill to that line.
Definitely stay a bit on the too much side, than too little, but it really doesn't take much. Do not submerge your jars!! 3 quarts of water should be ok. After it is up to pressure, just make sure you turn your stove down while still keeping the pressure up. For my stove this is usually between medium and low. The lower you can get it, the less water will be boiled off. The temperature in the cooker is the same, as long as steam is still coming out.
Keep any eye on it, in case you do run out, you'd ruin some jars that way. You want to catch it quick. The second time around you should have a good feel for how much water it takes.
