the best way to find out exactly how much liquid you need is to take a bum roll of crap film you'll never use (way expired drugstore c-41 is perfect), spool it onto a reel, put it in the tank on the spindle, but don't push it down to the bottom, let it sit in the "top roll" position...as if it were the fifth of 5 reels and put the tank on its side. Add something like 400-500ml of water and hold the tank (with the lid on) parallel to the floor and rotate it (or put it on the roller) if you pull out the film and the whole roll is wet, it's enough. Add on 50ml just to cover your bottom, and there's your figure.
For what it's worth, my jobo drum uses 270ml of chemicals for the 2x 35mm rolls drum. I would extrapolate that for a paterson 5 reeler (I've got one too), you'll probably need about 550ml.
I agree with that, probably a little more for the Paterson because of that wider top lip