The magic of probability is beyond me but I'm sure this is a very easy question for someone else.
Assume an attack from a single arc with only black dice in range. At what number of black dice does it become more effective to have Ordnance Experts to re-roll misses than to add more black dice with External Racks.
This is a question aimed at RitR ships with only one upgrade to start. I realise that OE has other benefits like it can be used on both attacks and over repeated turns and so on. But what if you were only expecting to get one attack?