i tried to research this answer, google and brewtalk and youtube, before i posted a new thread. i just realized that "spacing" was a good search word, and i found this. http://www.homebrewtalk.com/f11/cool...spacing-61815/
i used CPVC and this guy used copper, but he said he did a slit every 1/4" with a dremel cutting disc and the cut was cleaner than a saw. if it melts the plastic, i will use a saw.
i'm going to try to minimize the amount of liquid that gets left in the bottom of the tun when it is drained by cutting slits in the bottom of the manifold as opposed to the top.
i'm also going to not cut too deep into the manifold, because when the water level drops and you start getting air in the slits, your tun stops draining. i'm not going to cut more than 1/4 of the way through the pipe.
i'm also going to soak hot water in the cooler for a couple of hours, then drain it through the valve, and measure how much water gets left undrained in the bottom.