Update for anyone reading in the future...
Eventually after many phone calls, hole size and fan speeds are determined by the designer of the system using various calculations.
Turns out the problem we have is/was, the system was designed by one company and the parts bought from another. Both said its not our problem any more as you should have designed and bought from one source. Ahhh purchasing departments headbang headbang headbang
Anyhoo. We're installing, then passing dimensions back to supplier, who will calculate hole sizes and other parameters for the system, then we/they'll set it up.
What a joke. :coffee