What pump do you plan to use? Because I can't think of any pump that would run at it's maximum output pushing 800gph through 1/4" PVC. Depending on the pump, 3/4" or 1" PVC would be more ideal.
As far as holes go, start by drilling a few 1/4" holes (about 5-6 to start) in the the 12" section of the pipe that will serve as your spraybar. From there, take a look at the flow that you're getting with those holes and add more as it seems necessary.
Also, plumbing a union inline right before the spraybar will make it easy to remove the spraybar for this experiementation, but more importantly, it will make removing the spraybar for cleaning much easier in the future.
