I haven't experience yet, but I'm in a similar situation while wanting to make a distribution manifold for a return line from a sump. It may be that you'll need to make two, unless you get lucky with the first attempt.
Too few holes would unnecessarily choke the flow rating. Too make would lessen the velocity of the spray bar. My plan is to start out with some holes, then keep adding more while analyzing the impact and once the overall flow starts decreasing call it quits (or make a new one, with just a few less holes if you've gone too far).
The one thing I never see people do, is neck down the spray bar the further it goes. Home HVAC systems are plumbed in a way that the interior volume necks down the further you get from the pressure source, presumably to balance the output of the system. I've wondered if that extra detail would have significant impact ever in a tank setup. Then again, it'd most likely be blind guessing unless you take the time to figure out the formulas are do the math.
