The total area of the holes in the spray bar should be less than the cross section area of the tube. So, if the tube has a .5 inch diameter hole in the middle, and you want 10 holes in it, the diameter of each hole should be less than .5 divided by the square root of 10, or .158 inch. If you wanted 100 holes, they should be less than .5 divided by the square root of 100 or .05 inch in diameter. If the holes are a lot less than that, the water flow velocity out of the holes will be high, but this will cause the filter flow to drop due to the pressure loss. So, you just want them to be a little bit less.