Like the previous post, you will be able to drain off more wort with the slits face down. If you are using for instance, 1/2" pipe and cut halfway through, you will be leaving 1/4" of wort in the bottom.
Down. and the slits should be about 1/2 inch length spaced about 1/4 inch apart down the length of each pipe.check out my gallery for a good example of a copper manifold